Gus Potter

I would bite the bullet and get an SE in for a look at this. While there are a good few good competant builders out there.. there are less / few that have a true understanding of overall building stability, it's not really their brief so you can't knock them. The main thing is to recognise is that even though you may be looking at the vertical loads you need to hold up, the pillars and even innocuous bits of wall, may be providing horizontal stability to other parts of the building. Often you find walls that don't carry load from above.. non vertical load bearing walls. But these walls are still load bearing walls.. making a major contribution to the walls that are carrying vertical load. They prevent the vertical load bearing walls from buckling and / or to resist the wind loads that want to push your house over like a pack of cards. For all it will cost and in the interests of keeping you all safe, ask an SE. They may even suggest a cheeper way of doing it. If not you will have a correct design that you will be grateful for when you eventually come to sell the house.

Hello Hill Runner. Great place, high up so next stop is the moon. I'm down the road/ hill from you but have you bought a gold pan yet? If you have attended to all the gutters, down pipes etc and checked the ground levels are not bridging the DPC what about strapping the walls with 50 x 25 treated timber. Fix a bottom and top rail and verticals at 400 centres. Pack the straps off the wall say 5.0mm with a small cutting of DPC behind. How old is the house, does it have a bitumen DPC.. what is the construction of the external walls? Ah saveasteading, good point, made me think. To saw an 8 x 2 structural timber out of a 16 inch log after cleaning & stripping say requires four saw passes. But to plain saw a 2 x 2 requires another 3 passes of the saw? So more sawing to produce smaller timber out of the same size of log. But in real life the smaller logs are used for the smaller sizes. hey ho. The point is that you need more sawing? Also the surface area is greater relative to the cross section area of the smaller timber so you may need more preservative and a bit more handling? Maybe this goes some way to explaining why smallar timbers cost more than standard large ones per volume of timber? Thanks Dave for the Larsen truss, great spot and Steamy too and everyone else. Lastly to Mike, but setting aside a real good practical point from Mike and for a bit of fun I have had a look at the Larsen truss, how it works to some extent in terms of creating a twin wall so you can create depth for insulation. I've tried to capture some of the main points but..! Mark et al please chip in if you think I have missed something or made an error as no point in posting incorrect stuff. Here goes. I'll put some qualatative numbers to this. Work out the weight of the cladding, say another layer of OSB, battens and some timber shiplap. Shiplap thickness 14 mm. Timber density = 420 kg/m cube 0.014 *420 = 5.9 kg/ m sq Vertical 50 x 25 battens to create gap between board and OSB @ 400 centres. 0.05 * 0.025 * 420 / 0.4 = 1.3 kg / m sq OSB 9.0mm thick 5.5 kg / m sq. Sum of above = 5.9 + 1.3 + 5.5 = 12.7 kg per sq metre. Convert kg to kilo Newtons 12.7 * 9.81 * 0.001 = 0.12 kN add say 25% for "other stuff" like insulation and self weight give 0.15 kN per sq m.. not a lot as 0.15 kN is about 15 kg per sq metre. Over a 2.4m height this gives a load per metre run of wall of 0.15 * 2.4 = 0.36 kN ~ 37 kg. Now because the cladding is off centre from the point of support it causes a bending effect in the vertical section. The bending effect (the bending moment will be ~0.15 * 0.4(lever arm) = 0.06 kNm this is very small. For example if this was a domestic floor engineered I joist we design for the dead loads (self weight, flooring plaster board etc) and live loads, you and others. The live load is 1.5 kN/m^2 (~150kg) for a domestic floor so the bending moment is calculated for the uniformly loaded floor joist under live load space at 400mm centres as follows. Spacing * Load * length squared /8 =0.4 * 1.5 * 2.4/ 8 = 0.432 kNm which is more than the above bending force from the cladding weight. You can see that the bending effects from the cladding weight probably don't govern the design. Look now at how the cladding weight gets to the point of support. For every run metre run of wall we have about 0.36 kN..(36 kg) of cladding etc not a lot. There are battens at 400mm centres so each batten has to transfer via the nails 0.36 kN *0.4 = 0.14 kN over it's height. A very small amount. Now a 2.7mm diameter nail from 9.0mm OSB into a C16 grade timber has a shear capacity of some 0.05 to .08kN capacity. So the minimum number of nails will be 0.14 / 0.05 = 3 number. Again you can see that you don't need many nails to do the job to transfer the cladding load via the battens to the 50 x 50. We can set that aside for now. If using a cement board say we may need to revisit. But there is no free lunch! Firstly for it all to work the load from the cladding goes into the 50 x 50 but if you only have 3 nails up the height the 50 x 50 will bend like fury and it won't work. For it to work as a truss / I joist you are looking at nails at 100 - 150mm centres to connect the OSB to the 50 x 50. Then you are on the ball park. You may have a wind load acting on the cladding and this has to be resisted. Wind loads could be 1.0 to 1.5 kN/ sq metre so they have a big effect, much closer to a floor joist type live loading. This could often be the governing factor in the design. Lastly as @MikeSharp01 points out there are practical things that need consideration, how do you fit the insulation, what type and so on. Also, you want the 50 x 50 to be stiff, you don't want to be trying to fix cladding into a small timber that bounces all over the place. I think that although you can show that the forces can be accounted for structurally and you can "technically design" something lean you need to look at this in the round, particulary the buildability, labour cost and how easy it is to achieve a consistent quality of workmanship. I think this is the key to getting this to work.. is the practical buildability side. That said it's a great concept to consider if you want to achieve low heat losses.

This is a off topic but @SteamyTea makes some good points. Also if you look at @zoothorn posts, it's infectious, the sharing of knowledge, questioning and exploring. The enthusiasm of all is clear, everyone here is learning something, or just enjoying the collaborative thinking. Turning now to what steamy has posted, yes OT but here are my thoughts and how this enriches BH. As a bit of a back storey. I left school at 17, went to college and got an HND in Civil Engineering, worked for a few major construction companies and went self employed when I was about 22 -24 , a while ago. Built up a reasonable business as a building contractor and had a life change at 40. When I was a Contractor I always enjoyed the teaching side, bringing on the apprentices and making sure that everyone else was able to grow too. It was not easy and eventually.. Packed it all in 40 and went to University to study to be an SE. That with hind sight was probably the best decision I made... I still remember my first week at uni at and sitting there thinking.. these lecturers are giving me two things: The basics; maths, how to write and communicate etc but most importantly the tools that I need to enable me to teach myself that will set me up for the rest of my life. I appreciated this as I was older, not many kids can see this, and they can't be expected to either. During my time at uni I was fortunate to be invited to participate in reseach as an undergraduate, I was able to bring my commercial experience to bear which clearly most kids don't have.. and off the back of that I got involved in the "educational" side of things also.. and that is why I'm interested in what steamy is saying, but I'm also fascinated about how folk learn on BH, why the mods do what they do with apparently little recognition. Mods.. OBE coming your way? Steamy to quote you "Some educationalists think that courses should be developed to a more specialised areas i.e. Forensic Science, Renewable Energy, while others think a more general education is better i.e. mathematics, Physics." Steamy I agree with your view. One key for me in relation to tertiary education is that the educator.. you for example has to be skilled in the art and science of education and to be fully invested, in other words you need to be able for example to let the students see that you are enabling to teach themselves them not just delivering a lecture. Like all things in life if you can let folk see that you are doing something that is to their direct benefit then they will take notice. An educator of younger folk also need some good sales tactics! That is the true skill of an educator. I know this works as I have been lucky enough to be taught by some expert educators and have been old enough to appreciate what is going on. Now a skilled educator needs to be renumerated.. and recognised as a contributor to society.. we don't really appreciate this as much as we should in the UK. I could go on an on.. but Turning back to BH. It's a great site for all. No matter what stage in the "things to do with houses" you are at. BH is a great resource. That's it. but thanks again to @zoothornfor starting this thread

It does and your grass does not grow, also in my mind what is the point in saving energy if you chill the environment and thus inhibit the wildlife that we rely on. Could be the bees polinating the crops we all rely on for food, the insects that need warmth to thrive and the birds that feed on them. We should think carefully before we mass produce heat pumps and stifle the last bit / refuge for wildlife in our cities.. there could be unforseen consequences. That said though the recoverable / easily recoverable energy density? in water is a lot more than air. @SteamyTea not strong enough on the technicalities of this? can you help? @zoothorn the best place to recover heat would be just downstream of a neighbours septic tank.. just don't let on they are heating your cabin.

If you don't get your lay persons explanation accepted then a desk top study should do the trick unless you are next to a landfill etc as @saveasteading circa 3 300 - 400 quid maybe topsbut most BCO's already know if there is a potential soil contamination risk. You easiest way is just to make a phone call and ask what you need to do to satisfy this item... ask for help and it is often given.. get stroppy and it is less forthcoming.

If you want a bit of input.. often free professional advice then what about the check report from AIS so we can see exactly what their requirements are. Make it easy for BH members to see your dilemma and you may avoid having to get the cheque book out.

Any chance of a bit more info.. just curious.

Great thread you have got going here @zoothorn That stream / burn you have there. I wonder how much heat you could extract from that with a heat pump without damaging the wildlife. I used to live out in the country and had something like that. On my wish list is to have a go at a DIY heat pump, just for a laugh using an old fridge compressor in reverse, a few controls to reverse flow to get the automatic defrosting. Oh and make sure you install you flue the right way up all along the length.

A few photos of mine. Warm roof. Upstand 150mm, don't skimp on this as in the regs and also you get snow building up round about it, you also can get splashing from the rain.. funnily you can see this in the regs when they say a DPC should be 150mm above ground level. Over kerb size in the photos is 1420 wide x 2855 long. Upstand is framed in 95 x 45, 70mm PIR between and lapped with 200mm of PIR inder the EPDM covering so only 150mm is exposed at the 70mm insulation thickness. Opted not to do insulated plasterboard on top as it was closing in the aperture. Instead accepted that the upstand would be less well insulated and did compensatory U value calc and upgraded the rest of the insulation. Basically the lantern was so expensive that there was no way I was going to shut down the effective opening any more than I had to. Light is fantastic. Still need to do the roof of the main house.. soon!

Hello jayc89 I agree with George, quoted below. Resin anchor fixings, in fact any fixing has often has quite a lot less capacity than you think. You look at the fixing and think.. that looks sturdy..but it's the things round about that generally cause the issues. Some of these are: 1/ What you are fixing into. Concrete, natural stone, modern solid brick, modern bricks with holes in them, old say London brick clay brick.. a long list. The variation in load bearing capacity is enormous so please be careful. 2/ What you are fixing to the wall. Take a timber wall plate / bearer / ledger piece. The joists are connected via hanger to the timber, the load in the timber plate has to be transferred into the fixing. The timber starts to crush locally.. as you have a small bearing surface between the timber and the fixing. 3/ How much stand off do you have. You photo shows packers behind the wall plate. The actual behavoiur is very complex so will not explain in detail but touching on a few key points. The fixing deflects (bends) and this causes over stress at the fixing / timber interface. As the fixing bends it causes higher stresses where it enters the masonry as the fixing starts to act more "like a lever" rather than just acting in shear. One consequence of this is that the fixing develops additional tension and this causes other issues. 4/ Often the mortar is weaker than the masonry units / size of the stones if natural stone. The distance between the fixings and mortar joints is critical. For a bit of fun here is a real world example of how you design the fixings. I have screen shotted parts of the design. Let's take a house extension to a 1930's house.. old bricks, maybe some soft mortar. The key point is that for the fixings to realise their capacity they must be fixed nearly the centre of the brick and well away from the mortar bed. You can't control the floor height and you can't be certain that the brick coursing will be level. Thus you have to make sure that luck is on your side and you do this by adding in extra fixings. It gives the builder (maybe you) a fair chance to get it fixed safely, not slip later cause cracking, squeeky floors and so on. Remember that the new timber will shrink so while it may appear "rock solid" at the install it won't be quite so when it all dries out in a year or so. Example: Floor joist span 3769mm, joist spacing 400 mm The above gives the load on each joist. 0.81 kN/m is about 80 kg/m run of joist. Each end of the joist is supported by a hanger attached to the wall plate. The dead load is the self weight of the floor. The live load is you, furniture, book cases, having a party and this equates to about 150 kg per square metre of floor This is a code requirement that BC expect to see. The above works out the maximum spacing of an M12 fixing in a typical 1930's brick with the plate hard against the brick face. Each fixing can carry a permissible shear load of ~ 157 kg. But the test data is based on a European masonry size, not an imperial size! Thus while we can adjust do other calculations we apply some conservative Engineering judgement. The result is a detail like this. The detail below is a ground floor, vented solum below with PIR insulation. You can see that in actual fact you need quite a lot of fixings to make it fly and safe for the design life of the building. If you have read this far then thank you and I hope it helps.. a bit.

Just wondering.. did your neighbours extend first? Is the outside of the brick on the boundary? Do you know anything about the founds, do the the founds as existing encroach onto your land? Who owns the existing wall? That open cavity.. is it ventilated or just as a result of bad workmanship? Are you adding any load to this "party wall" ? To design this I would first look at how you can do something that does not interfere with the existing wall in any way. Thus you can't be blamed for anything later on. Avoid cutting in DPC's, chemical DPC's are a waste of time and also you need to drill holes in the wall.. you could be blamed for any cracking / the smell of the stuff permeating next door.. and so on. Sounds like you get on ok with your neighbours but a quick dilapidations survey may avoid any potential issues in the future. Quickly.. I would explore this. Clean the face of the existing brick. Apply SBR bond then when tacky a cement screed 10mm thick with waterproofer, then another the day after, then a cement based water proof polymer slurry.. timing is the key here as you want the renders and slurry to set but not cure fully. Take it up say 150 - 300 mm above the neighbours floor level as you show. Now strap and line the walls but don't fix though the renders at the bottom. Keep a gap between the timber and the masonry / screed of say 50mm. Make sure you isolate the floor slab from the wall using 50mm of vertical insulation, rather than what you often see as 25mm perimeter slab insulation. Keep your DPM on the inside of the perimeter insulation and once the tanking has dried out tape it to the slurry.. that is kidology as that is supposed to stay glued for 50 years but it won't.. looks good on the drawing though. The main thing though is that provided you keep a bit of heat in the wall all will work... the damp won't rise particularly if the neighbours extension is a fairly modern brick and the dew point / thus evaporation won't occur thus drawing further moisture up from below. Maybe go for 30 - 40 mm of insulated plaster board / vapour barrier as a backup. Here you want to actually loose some heat to keep the party wall that little bit warmer. Doing so will mitigate condensation (the dew point) = rising damp. Compensate for this heat loss by improving insulation else where as this is easier to do and often more cost effective. Does that sound like it may fit with your set up?

Definitly not. For me the cross fertilisation between different disciplines is inspiring. Thanks again.

Steamy. Just a quick note to say thanks for your great posts, really enjoyable reading, learnt a lot from them so thanks. Also thanks to everyone else that has chipped in.. it's a long list but thanks all from me.

Yes, sorry to confuse. I put up that screenshot to show what the perimeter insulation looks like, not appreciating that there was dot and dab above. Agree with Tony, strap and line. Dotting and dabbing onto old walls can sometimes cause patches of condensation. All the best.

There was a whole page on swimming pools that cropped up. Not a big issue up here in Jockland as it's wet enough already. Have a good PI broker that understands the construction industry so can discuss risk and what I'm up to. Yes but you hit the wall when less than 1.0m from a boundary both under the English and Scottish regs. Strangely fires in Scotland are suggested to be hotter than English / Welsh / NI? fires once you really devle down into it, but at that level much is open to intrepretation and the wording of the regs. I'm curious as to the 40% value. Is that something to do with the regs or a rule of thumb when trying to get off the shelf insurance, NHBC type support if you are a volume house builder? I don't think they do as it's not a mass market. Hopefully things will settle down and lenders will appreciate that there are "new" proven ways of constructing houses and that there is money to be made by writing policies that cover what we are discussing.

Hiya. If the floor screed touches the outside wall you get a cold bridge. What you do is stop the screed often 25 to 50mm from the wall and put a bit of insulation that is vertical. Like below from screen shot of Kingspan detail To finish the existing house walls I always want to know if the existing wall has a clear cavity or if it is fully filled with insulation. If not fully filled then you often get a cold draft in the cavity so the existing wall is cold. Plastering this is not my preference as it can attract condensation at the bottom / top if a kitchen area. My first choice is to strap and line with 40mm insulated plasterboard. Yes it thickens the wall a bit but you can lose quite a lot of heat on a windy winters day due to a drafty cavity! I would look to see if you can tape and seal the DPM as best you can, avoid raggling (grinding a groove) out the block to rebate in the plastic.. not a good structural option over that length of wall. Try and lap a bit of DPM up the wall, tape and seal and take it above the top of the screed. The vertical insulation (perimeter insulation) is often water proof anyway so you have belt and braces.

There are lots of reasons for doing this even if not absolutely speced by SE. Yes the adding a bit of OSB to the wall adds robustness to the building. Take photos if you do as if you make alterations later then you may win a watch. Make sure you use the right nails. Say 3.1 x 50/65mm long improved of galv nails at 150mm ctrs around the perimeter and 300mm internally, standard TF spec. On a day to day basis great if you want to have something to say fix like shelves. Acoustically you may be better with two layers of plaster board, but it will help a bit. Again in TF design we sometimes are allowed to take account of the stiffening effect of plasterborad, ok but I'm not that keen say near bathrooms as it gets wet. The main thing is if you want to add stiffness from the plasterboard is to close up the screw centres, use screws not nails as you can get better control over the fixing indentation in my view. In any event always worth improving a buildings robustness. Just one last thing.. if doing so in a corridor or up a stair say make sure you don't accidentally make it too narrow and breach the building regs by 11.0 mm!

I think Charlie has a point to make in the context of the current climate about obtaining insurance for self build.. at a reasonable rate. Charlie.. my interpretation.. please forgive if wrong. After Grenfell, we have not made any material progress in terms of updating the regs to date. When last renewing my PI there were about an extra five or six pages that asked in great detail about what I have been doing in the past and what I will be doing in the future, and even what I may be thinking about in terms of cladding and fire design. I almost expected that the last question would be on my preference now or intended of underware.. boxers or something more.. modern, body sculpting but flammable. Clearly the insurance companies are evaluating their risk model. Charlie that said I hope that you will agree that we can often design a TF with significant areas of say timber cladding that can comply with the regs even if it is more challenging to obtain insurance in this current climate. Once you get close to the boundary of a site timber cladding often just does not fly as an external covering, but in general can we live in hope?

Delighted for you, great part of the country. I'm not far away from you. As others have said planning is to do with what it looks like from the outside not what is behind the facade. However the planners may be quite interested in what you are doing as they will may want to learn from what you are doing for experience. BC will also I think be interested for the same reasons, when you get to that stage. My advice is to engage and embrace.. you may get a pleasant surprise, if not you can battle away after. In terms of the ground you may find... mostly over consolidated boulder clays, good ground. Occasionally if close to the coast then you may have what are called raised marine beds, these are sands and gravels that were left behind when the sea leavels were higher after the glaciers melted.. look up last Loch Lomond ice age etc. Now you also may have a bit of mining. Follow link below for a play about. https://mapapps2.bgs.ac.uk/coalauthority/home.html Your Cruck frame. Spot on. Don't know how much you know about these but even if you know a bit go back and look at first principles and how they work. The key parts are the cruck blade, collar beam and arch brace. They are essentialy how a modern portal braced frame is assembled. Have a look here at a knee braced cold formed steel frame.. see the similarity? https://www.capitalsteelbuildings.co.uk/gallery-agricultural/ If you can get a handle on how the main frame works then you are up and running. The other parts of the frame are there to add extra stiffness to the main members and act as a carrier for your roof and bales. The main thing for me to say.. hey the roof is permenant, provides lateral stiffness against wind loading parallel to the ridge and we need some other logitudinal bracing in the direction of the ridge within the walls. Try and put all this on the warm side of the insulation envelope. Then introduce the bales.. really they are just like rendered wool / wattle / carrier boards. The main thing is if you get it wrong with the bales you don't lose your shirt while getting something that will last many generations. Not sure how far on you are but what info do you have at the moment? Don't rush to trial pitting before you have got all the other stuff in place, no point in just digging randomly before you have got the planning sorted and the footprint established. You may accidentally dig a pit right under a frame base, soften the ground and cause yourself a problem. Sometime pits can be targeted so you can also use them to gather info on soakaways etc.. get more bang for your buck. All the best and keep us posted.

Well @PeterW @CharlieKLP You have opened a can of worms here! I think Charlie is right to some extent that you may find it harder to get a range of offers on a self build loan if you have what the insurance companies deem to be a bit off piste, not enough info on the risk. Disagree with Charlie here, it's a relaxed forum and sometimes we just choose the wrong form of language.. I often do. @Selfbuildnewbie To help you on the way. A timber framed house is just that. The internal timber frame carries all the loads, from the sideways wind and the floors and so from above. This is the skeleton of the house. Attached to this can be, facing bricks / blocks / timber cladding / cement based boards that are finished with something, could be paint even or thin texture material. It's not quite that straight forward from an SE point of view as the outside skin can also stiffen the TF so the outer skin does have some interaction with the TF..long story. Now wood burns, thus if a fire starts on the inside of the house you need to stop the timber frame from catching light from the inside for a period of time. On the outside you have to clad the frame with something that will not do these things: 1/ To not become alight if say a window breaks, sparks / flames fly out and set light to the outside of the cladding. This is called often surface spread of flame. Thus you can't have on outer skin crepe paper, to be extreme. 2/ That if you get visited by vandals it's not easy for them to set your house on fire.. again surface spread of flame. 3/ That the outer skin of your house will not heat up too much / or fail and send sparks over a boundary. Long story but this is called a fire boundary condition. If the wall heats up it will radiate heat like an electric fire. Sparks are self explanatory. 4/ That you protect the timber frame skeleton so that it stays up long enough and not fall down as it is holding all the other components attached to it that are doing their thing. Funnily you don't have to protect your detached house from a neighbour's fire under the fire boundary regs. However I always like to think about how you protect folk from the daft things their neighbour may do. @Selfbuildnewbie don't be disheartened.. it's a big challenge to self build but you are going about it the right way. So much info to process... it will all come together in the end though.

OT sorry.. It's a bit of a scunner really that the insurance companies have these quirks. It's perfectly doable to design a fire resistance low rise TF house structurally that is also fire safe. It's a huge topic but the main principles are: 1/ You need to design so that in the event of a fire the occupants are warned of an outbreak, that they can exit quickly and safely. Should they not be able to do so then they are protected from the effects of smoke and heat for a length of time sufficient for the fire service to effect a rescue. 2/ That the fire service are not endangered by an unexpected / disproportionate / rapid collapse of a structure. 3/ That you don't set light to neighbouring properties. In most cases I think the biggest and most common bill the insurance companies need to swallow is that from water damage caused caused by the fire services on low rise housing. Hopefully things will change in the future as insurance companies revisit their risk exposure. Yes the dormer cheeks are part of the roof so often considered as a non structural element.

Bit of SE input here just to complicate matters! When the wind blows it endevours to collapse your house like a pack of cards. To provide a bit of context. Even in benign parts of the UK you are designing for a magnitude of 200 -250 kg of force for every square meter of upwind elevation exposed to the wind. You can quickly see that the sideways load is quite a lot, given that say a family car may weigh some 800kg? Also the roof.. it is quite a big house and the roof acts like a sail on a boat.. more force. Some of this sideways load is resisted by the outside walls but when you put big openings in them they don't work so well. Thus to offset this you want a good few walls internal walls on the ground floor in both front to rear and gable to gable directions. Ideally you want walls that are longer than they are tall both internally and externally. This is a good way of grasping what will be an economic structural concept. At first glance your ground floor layout looks like you will need to spend a bit more on the structure, possibly more than you want. The pocket doors are shown within the thickness of a standard wall.. tricky. If you look at how much wall you have left that is available to turn into a structural wall then you don't have much to play with particularly in the gable to gable direction. You can still have pocket doors but often you need to thicken the wall so that one part of it still remains a structural wall that can carry both vertical loads from the floors above and resist the wind loads. You have some pretty big spans there on the ground floor and a fair number of things above to hold up so bearing this in mind.. I would ditch the pocket doors from an SE point of view. Also in service.. having pocket doors from say living room to a music / tv room does not stack up due to sound transmission. It's great the open plan idea but they can be noisy. It's time to put some gridlines on these drawings at the concept stage so you can see how things line up properly. You could easily loose thousands here by needing extra structural works that are not required!

@JohnMo That does look good I think, once it's planted up and weathered a bit..better still. Does not look like you are retaining the full 2.0m height on the high side though so probably not that much load on it. Could be a haven for insects and solitary bees especially in the winter as the soil at the back keeps things a bit warmer.

Good point.. but remember that you could be weakening the existing house wall in terms of horizontal stability if you cut it. I would check with the designer before you do this. @jimal1969 where the new wall interfaces with the existing rear wall looks like it is in a pretty sheltered spot so less risk of driving rain causing a damp problem. I would try all other options before cutting the existing brick first. I would imagine that you are going to strap the existing rear elevation wall and line with insulated plasterboard anyway so the any occasional increase in mosture is not going to pose much risk. If you are just going to plaster onto the existing brick then.. could be an issue for condensation and so on. In terms of you DPM. Are putting solid insulation on the slab with screed or some kind of floating floor? You usually need some perimeter insulation vertically so should be able to cobble up something with tapes and some extra DPM / DPC to protect everything from the masonry below DPC level. Anyway you are now into spring.. good building weather.. what more could you ask.

Good question about restraint. These brackets are good but does this seem possible? Build the 100mm block off the 140 block. Each side put full depth noggings (dwangs) hard against the 100 mm block within the depth of the floor joists. The noggings will also serve to support the chipboard flooring edge and the noggings are then tied into the first floor diaphragm. Even what you have is recognised as an effective lateral restraint so the noggings will really do the business. The 140mm block, although a pier looks suspect at first glance, will carry a lot of load as the joists above are loading the wall almost about its centre of gravity (photo 2). As the joists above are continuous the wall will attract more load, which acutally helps in a lot of cases to improve its resistance to lateral load.