Solutions

Dear NickFromWales, You do realise that you have just argued against probably 80% of the builders and architects doing basement retrofits in London ? Not that it would be a bad thing

Hi Russel, The expanded EPS has relatively good "breathing" (air permeability) properties, that's is why is actually makes a good pair with full walls and mineral finishings.... And also why they were soaked in smoke The extruded EPS (high density one) doas not "breath" and it is ideal for in insulating foundations, slabs, basements etc. And they do last forever

Hi Jones, If the concrete and earth layer are only 15cm thick ... JSHarris is right, it would worth more to unearth it and work everything from outside. (Apologies, I thought you had some serious ground on top of your roof) In regards, to the temperature debate, the 4C i uses for reference was from my exp with London clay (see the image attached). I have to admit that I am not too familiar with Wales soil (besides the nice sandy beaches) A lot of info and interesting reports can be found on the BGS website, including info about the thermal conductivity of various types of soil. C012 (1).pdf

... If the money are there, YES, ideally you would want to unearth it and do everything from outside. (See the first suggestion made, before the pics were posted) The problem is the costs, JSHarris, can tell us from his own experience how fast ground works cost can escalade. That is why i suggested treating the existing structure as a "retention chamber" and work from inside as on a traditional basement. The floor is the easiest part to sort, because by default he is forced to introduce a new layer (ideally full slab) on top of the existing one to connect with the extension areas (south and east) and to accommodate the water proofing and drains. The thickness (raise) that will be imposed by the drain depth can be filled with high density polystyrene (like you use in a resting slab). In regards to braking the thermal bridges, you can do a reverse brake from inside like you do in retrofits. Yes, it will eat up space by thickening the columns by an extra 10cm , but it will be way cheaper then unearthing. Plus, if the ceiling is under the freezing depth (60-80cm for UK if i remember correctly), the earth layer should provide a natural insulation with a relatively constant +4C during winter time. Personally I would always go for external insulation and proofing, but sometimes the deciding factor is the money

I look at the pics and the caveman in me awakens ... Honestly, it looks amazing and from the pics it has huge potential. 1. What is the foot print ? 2. How high is the ceiling at this moment ? 3. Have you decided if you will unearth it or work only from inside ? 4. How will you sort out the natural light? It does not seam to have enough hight to allow for light tunnels. If you have no points of infiltration, it will be cheaper not to unearth it and just work from inside. - sika on the concrete, or waterproof membrane, or both - drain membrane, with a drain canal (and sump pump preferably) - for the inner leaf, depending on budget, either stud with build in insulation, or thermal block with EPS. Regardless if you chose stud or block, for the termal insulation I recommend you to look at polystyrene (specially at the graphite EPS). I know its common in UK to use mineral wool or Polyisocyanurate (kings PIR) but in my opinion they do not behave that well on the long run if exposed to humidity. And a suggestion about the pillars, clean them well and dress them up in stone. They will look nice and also act as a thermal mass. For the beams, you can find a good carpenter to dress them up in wood, ciseled, burned, stained and lacquered to look like an old single piece.

ProDave, For the infill, i agree it is an elegant solution to pay by cubic meter or linear meter of run, but not when the client wants a price set in stone. For the cable surprise, think of it from this perspective ... If you discover a wire where it should not be, you call the DNO and they move it where it should be. In this case it delayed the start by 4 weeks but it was done at the DNO expense because it was thier fault. If you dig and and you spot it before you hit it, you will have to stop the site to investigate and do a site survey ... If you consider even 3 days delay x £1500/day =£4500 plus a new survey ... If you dig and you hit it, you support the costs of the repairs because you did not take the precautions. Not to say that hitting a HV cable usually ends up with HSE on your back in the best scenario (or God forbid, with someone not getting home from work)

Let me guess, probably 50% of the 50k spent, went to disposal and landfill tax ...

Completely agree, the survey costs that i usually incur are higher then standard but on the up side they are way cheaper then having surprises later on. Couple of years ago i refused to start the works and i asked a client to spend £6700 on surveys. The main contractor and the architect went ballistic on me and threatened to terminate the contract ... They both calmed down after the ground penetrating radar showed that a main electricity supply was 2,2 meters away from where it should have been according to plans, and just 40- 60cm from where i was supposed to dig and get the level concrete in Not to mention a pocket of rubbish infill, on the right side that forced us to go deeper with the foundations. I have to admit, i could not help myself and i have rubbed it in their face

True, unfortunately so true. A couple of months ago i was talking with an architect. He mentioned something i did not new ... In 2012 or 2014 (cant remember exactly) the government encouraged the councils to give the plots to developers under a BNPL scheme. Basically they got the land but they only have to pay for it AFTER they build and sell the new properties. The result, tons of land stuck in to land banks that cost developers literary nothing to keep but brings them huge profits if they postpone building.

Hi JS, I agree that the higher risks are on ground works, but you can easily eliminate the surprises. I give you an example as a PM on slightly more complicated job. - got in a serious set of surveys and literally scan every single square meter - (never trusted the paper information, too many times ive found ground pipes or cables where there should not be). - soil survey over the top with bore holes every 5 meters - proper shoring and piling - 2 serious mud pumps Result, 110sqm basement, 3.2m high with 4 sides piled wall 9m deep. Total cost £176k waterproofing included and 22% margin ... The budget allocated by the architect £320k. Like i said, provide all the information to the builder and he has no reason not to accept a fixed or firm price.

Similar question ... How did you get CIL exemption with out planning ? Build term, if you have a good builder, 500sqm, from when they get in to site till completion, 7- 8 months tops, for a traditional build. 5-7 for a continental full wall. If you want to finish it in 4-5 months, negotiate with the builder for payment at completion of works and a pay-up the hotel bill if there are delays ... That will make them Speedy Gonzales Fixed price, no problem, have a good BIM with a detailed 3D that leaves no room for interpretation and you get yourself a fix price. As long as you give the builder ALL the details, it is his fault if he "missed" stuff. If you want freedom to change your mind about finishings, negotiate a fix price with allowances, for finishing materials. Example, if your allowance for tiles is £50/sqm and you decide on a £40/sqm you get back £10, if you want a £60/sqm, you pay up an extra £10.

Hi Jons, I am not familiar with how your tank was build, some pics would have helped. Normally i would treat the existing structure as a retention wall. Idealy looking to have watterproofing on the outside of the existing structure and an internal leaf with thermal insulation, secondary waterproof and drainage build on the inside. For the base, will probably be forced to add a watterproofing and drain system on top of the existing slab of the tank and get a new slab on top of it. If what i say makes no sense pm me and i will send you some detailed drawings and pics of some basements done inside retention walls.

Hi Ragg, Congratulations on the new home and thank you for the advice. Your route make sense if I manage to spare the extra money implied, because it will theoretically allow me to find a plot easier. Unfortunately I am on a tight budget , paying for an existing house that i have to take down to make room for the new one, is not that cost effective unless i plan to sell, not to live in it. But there is an extra risk... One of my clients has tried a similar approach in Barnet, old run down house, 2 floors. The council denied the demolition on the grounds that it will ruin the character of the area ... Lol. But they did approve a back extension and a side extension ... The complexity of the extension works was so bad that it cost him more then what it would have a cost a demolition and new build.

One year down the line and couple of council meetings ... Nothing, nada, zero ... To make things worse, as soon as you find some suitable brown land that was forgotten, and ask the councils about it ... Either some well connected developer will grab it from under your nose ... or the council realises that they have an asset and puts it to action ... With the same result.

Is self build an alternative for first time buyers in london ? YES Why is self build cheaper or better ? For so many reasons. Example ... 2016, Romford, guy buys side garden for 90k. Gets planing permission for a small foot print 4 bedrooms (8.5mx6.8m). Planning architect, convenants etc, 15k Gets a builder, Build cost 156k. Total, 261k for a 4 bed with a small garden. If we consider that the 3bed and 4bed old houses on his street are sold for 390-430k or a new 2 bed flat goes way above 300k... It is easy to understand why a self build would be better.