Selfbuild Experiment

[Zak S]

Hi I am starting on Selfbuild Journey. We recently bought a banglow built in 1960s on a very clayee place with existing foundations between 400mm and 1m. There has been no movement/cracks. Good old days..... but it wont pass the building regs these days and we want to build on top. Hence, starting again is the option we are seriously considering to future proof the house to some extent. The existing banglow is c180sqm and with extension we can go to 225sqm plus if we build on top the first floor and the loft, one can see the expected size. 

 

This is a great forum and it has been very helpful so far and now I want to start active conversation with you guys who have been on similar journeys.

 

Is there anyone who can share the excel spreadsheet for the cost estimation. It will be a self managed project with core skills arranged with each of the skilled trade.

 

I read somewhere on this forum about Abbey pynford for foundations involving Piles. They have been contacted and yesterday the ground investigation work was done (expect the depth of piles to be around 10/11m with slab and the void to allow for the clay movement. That would a significant spend.

 

The reason I am describing this as an experiment is as I want to test getting material and fit out outs from manufacturer and the scale of the project is such as that in my experience manufacturer here or in Europe/China will happily provide the requirement. In other words what would be the bare cost of a house build taking all profits element within the supply chain and building contractor out. I am just keen to find that out as the build cost per sqm does not work in my head. I want to know the real breakdown.

 

So procurement is where the true experiment sits. I have some contact in China supply chain so any procurement from China should not be an issue. My plan is ensure the structure is all based on BBA approved items so that it qualifies for the warranty.

 

Key steps I have outline in my plans are as follows and I am expecting different skills are required for each of the key stages hence would source these:

 

Surveys / planning work

QS costing

Warranties

Procurement 

Foundation / ground drainage /Subfloor

Underfloor heating

DPM

Brickwork/Block work/steelworks and insulation 

Timber work

Roof work (clay tiles or sate -chinese/spanish)

Windows

Weather tightening

Heat Plumbing and in house drainage

Electrics first fix

MvHR

Drywalling (metal furring channels and then either tape and joint using plasterboard or MGO) trying to skip the wet plastering but enhancing the sound insulation

Stairs and balustrade 

Bathrooms

Tiling and flooring

Kitchen including separate scullery

Internal doors and skirting (composite skirting boards)

Decoration 

Second fix electrics

Lighting

Patio

Landscaping

Epc

(Any thing I missed -please advise)

 

I dont envisage I would be able to do all in one go so happy to divide the project in phases if required.

 

Would really appreciate any help/feedback/insight that can be provided.

 

 

Edited December 15, 2021 by Zak S

[SteamyTea]

Make sure you have around 300mm of floor insulation (check numbers against R-Value and concrete thickness).

You can't realistically improve that once the house is built.

[Zak S]

29 minutes ago, SteamyTea said:

Make sure you have around 300mm of floor insulation (check numbers against R-Value and concrete thickness).

You can't realistically improve that once the house is built.

Hi thanks for the advice. Is that under the slab or over the slab and under screed? I have been looking at my insulated slab but not sure if Abbey Pynford would accommodate that. I might switch to Ring and Beam piles and then block and beam which is traditional and different from slab foundation offered by Abbey Pynford. I wanted to avoid thermal bridging of walls to foundation slab. Not sure what other ways are to avoid this other than insulated slab?

Edited December 15, 2021 by Zak S

[SteamyTea]

1 hour ago, Zak S said:

Is that under the slab or over the slab and under screed

Under the heating system.

 

There are a number of slab/flooring systems that reduce the thermal bridging. Some will depend on your wall construction.

[epsilonGreedy]

1 hour ago, Zak S said:

Hi thanks for the advice. Is that under the slab or over the slab and under screed?

 

 

He is talking about the high-end of Passiv House trendy foundations. Still a bit niche but growing in popularity. Your average local builder will look confused if you suggest this type of super insulated raft foundation.

 

150mm of rigid PIR floor insulation is pretty good but a bit more pays off if fitting underfloor heating. 200mm is way better than most new builds, 300mm is phenomenal but only makes sense if you apply the passiv house ethos to the rest of the house structure.

 

What is you budget for this build? I think it is relevant before you decide to knock down the existing large bungalow.

 

Has an expert told you the existing foundations won't satisfy a building inspector.

Edited December 15, 2021 by epsilonGreedy

[Zak S]

8 minutes ago, epsilonGreedy said:

 

What is you budget for this build? I think it is relevant before you decide to knock down the existing large bungalow.

 

Has an expert told you the existing foundations won't satisfy a building inspector.

Thanks. Definitely not looking for Passivehaus. Piles with ring and beam would cost same as pile with Raft so thinking of going with Raft option if more economical. 400mm foundation is not going to work 10m deep clay and high water table. 

 

In terms of costing it will go through all that and a bit more before knocking down. Given the plan for procurement and self managed project management traditional per sqm cost would not be appropriate hence need the excel spreadsheet if anyone ca  kindly share that would be great.

[SteamyTea]

9 minutes ago, epsilonGreedy said:

300mm is phenomenal but only makes sense if you apply the passiv house ethos to the rest of the house structure.

I beg to differ.

Assuming UFH running at a mean temperature of 32°C, that will have a ΔT of ~23°C during the heating times, walls, which are generally of a smaller area (this does depend on design, number of windows and doors) will probably have a ΔT of ~15°C for the heating season (does depend on where you live and how hot you like your house).

[Zak S]

1 hour ago, SteamyTea said:

Under the heating system.

 

There are a number of slab/flooring systems that reduce the thermal bridging. Some will depend on your wall construction.

Thanks. That's good to know. I will look into it will be included in the building regs drawings.

[Big Jimbo]

Abbey Pynfold do piles with a reinenforced slat connected to the piles, with a void underneath. There is no insulation in the slab, and that has to be put on top of the slab before screed. I have looked at it as they are close to me. Not cheap though. I did also look at another company who use screw piles. These are all linked with a steel frame. Is insulated between the steel frame, and has steel sheets on the top surface. Cheaper, but i'm not sure how a warrenty provider would view it. Have you considered a basement ?

[SteamyTea]

4 minutes ago, Zak S said:

I will look into it will be included in the building regs drawings.

I think the regs are soon to be updated because of low temperature heating system that use UFH.

But the regs are a basic minimum and should be exceeded at all times.  The marginal cost is very small.

Too many people get hung up on windows because 'they want natural light', then have to buy blinds, screens, shutters and films to reduce the overheating.

Edited December 15, 2021 by SteamyTea

[Zak S]

26 minutes ago, SteamyTea said:

I think the regs are soon to be updated because of low temperature heating system that use UFH.

But the regs are a basic minimum and should be exceeded at all times.  The marginal cost is very small.

Too many people get hung up on windows because 'they want natural light', then have to buy blinds, screens, shutters and films to reduce the overheating.

Yes I agree with that that the minimum standard should be exceeded and that's what I'm planning.

 

I will slightly diagress here. In the same street they are two houses which are being rebuilt by turn key builders and the both builders are cutting corners wherever they can in terms of using material which just barely meet the standard. For insulation my understanding is that partial cavity is the way to go with PIR insulation TW10 whereas these builders are you using glasswool and not even properly filling the cavity. So I intend to ensure that I do exceed minimum standards but the cost is also the key driver for me. Frustrating issue is that one can get lots of building material from the overseas manufacturers much much cheaper and this is of good quality but the construction industry's rules are as such that the barrier to entry is significant. I have tried to go down the road of sourcing material internationally and getting it tested in the UK but but that is not viable as the cost of testing is significant in hundreds of thousands of pounds therefore I'm planning to make sure that the structural fabric of the property is all BBA approved so that there are no issues with warranties (though, I do understand warranties are not worth the paper they're written on but they are essential for the financing of the properties).

[Zak S]

47 minutes ago, Big Jimbo said:

Abbey Pynfold do piles with a reinenforced slat connected to the piles, with a void underneath. There is no insulation in the slab, and that has to be put on top of the slab before screed. I have looked at it as they are close to me. Not cheap though. I did also look at another company who use screw piles. These are all linked with a steel frame. Is insulated between the steel frame, and has steel sheets on the top surface. Cheaper, but i'm not sure how a warrenty provider would view it. Have you considered a basement ?

Yes I did consider basement and wa taken away from it by the architects whoever's I spoke with basement cost would be c £2500 or more per sqm hence I dropped the idea. I thought it would be same as cost of piling /slab but it was lot more expensive. Do correct me if I am wrong. Piling is just drilling holes which is lot cheaper than excavating the entire site.

[SteamyTea]

3 minutes ago, Zak S said:

I will slightly diagress here.

with what?

[epsilonGreedy]

47 minutes ago, SteamyTea said:

I beg to differ.

Assuming UFH running at a mean temperature of 32°C

 

 

A bit on the high side, 29 degress would be a reasonable temp to model with.

 

48 minutes ago, SteamyTea said:

I beg to differ.

Assuming UFH running at a mean temperature of 32°C, that will have a ΔT of ~23°C during the heating times, walls, which are generally of a smaller area (this does depend on design, number of windows and doors) will probably have a ΔT of ~15°C for the heating season (does depend on where you live and how hot you like your house).

 

 

Ok but now express that in extra heating ££'s per year for 200 sqm of internal floor space for both 300mm and 200mm of insulation.

[SteamyTea]

Just now, epsilonGreedy said:

A bit on the high side, 29 degress would be a reasonable temp to model with.

That depends on how much is lost to the ground, less insulation, higher temperature needed.

1 minute ago, epsilonGreedy said:

Ok but now express that in extra heating ££'s per year for 200 sqm of internal floor space for both 300mm and 200mm of insulation.

I am happy to cost things out, I charge a very reasonable rate.

[Zak S]

4 minutes ago, SteamyTea said:

with what?

Sorry ignore please. It was about my rant on the costs

[epsilonGreedy]

1 hour ago, SteamyTea said:

That depends on how much is lost to the ground, less insulation, higher temperature needed.

 

 

Nope the temp of the slab is principally determined by the heat loss of the main house in the case of the OP. The extra slab temp required to cope with the extra loss of heat through 200mm of insulation compared to 300mm is very small.

 

I recall that when using Jeremry's heat loss spreadsheet for 150mm of insulation,  ground heat loss was 8% of total space heating without UFH and 12% with UFH.

[SteamyTea]

2 minutes ago, epsilonGreedy said:

I recall that when using Jeremry's heat loss spreadsheet for 150mm of insulation,  ground heat loss was 8% of total space heating without UFH and 12% with UFH.

OK, you know best, can't be arsed to have to explain to you why you are wrong.

[Buzz]

Hi @Zak S this an exciting time and also one of the most important to get right ,over commit now and face the agony of trying to raise extra funds or compromise on the final finishes,  play it too safe and you may always regret not building what you really wanted.  

 

[Zak S]

1 hour ago, Buzz said:

Hi @Zak S this an exciting time and also one of the most important to get right ,over commit now and face the agony of trying to raise extra funds or compromise on the final finishes,  play it too safe and you may always regret not building what you really wanted.  

 

Hi. Thanks. Totally agree. The way we are looking at it is that we want to get the design and plan absolutely right hence going with a really good architect who has already done two houses on the same street. In terms of structure, we will play it safe (other than windows which we want to Internationally procure from China using the suppliers who supply a lot to Australia).  We are splitting the shell into sub and super structure appointing key skilled trades peoples for each  (sub) element. In terms of fit out we will experiment via cheaper procurement but use skilled labour. It would take a lot of time lining up these and planning before the project starts. Any tips would be very appreciated.

[Gus Potter]

9 hours ago, Zak S said:

Hi I am starting on Selfbuild Journey. We recently bought a banglow built in 1960s on a very clayee place with existing foundations between 400mm and 1m. There has been no movement/cracks. Good old days..... but it wont pass the building regs these days and we want to build on top.

As @epsilonGreedy says have you consulted with someone who knows a little about the ground?

 

Hope the following gives you some food for thought.

 

By all accounts the existing bungalow is sitting on competant ground. The good news is that for many domestic houses clay can be a good medium to build off. One early thing to do is to identify the type of clay you have. Starting with the basics. Have a look at where you live and look at how far the last ice age extended. Ice is heavy and has often pre compressed the clay we encounter today. We call this an over consolidated clay and this can carry a good bit of load without excessive settlement. You may have this under your bungalow. Dig a hole down to foundation level and have a look at the clay. Is it like pottery clay that you can easily mould and press your thumb into with little effort? or is it a mixture of different sized particles and harder to indent and mould? Grab a copy of the "Structural Engineers Hand Book" F. Cobb and you find info on how to relate this to the soil bearing capacity. You'll find same info else where also.

 

Spend some time on learning about clays and you are on your way to saving money!

 

The next step is to ask.. ok we want to put a heavier structure on roughly the same foot print. Can we just add more strip foundations similar to what we have and put something under the ground floor that will spread the higher loads over more strips? You can back calculate (on the back of a fag packet) the loads the bungalow is imposing on the ground and compare this with the weight of the extra storeys.

 

There comes a point where you need to many strips of founds. Practically they become so difficult to set out on site, dig with a machine, clear away the excavated soil and pour that you abondon the idea. Technically the bulbs of pressure under the strips start to interact and you are faced with the law of diminishing return anyway.

 

The next stage is to say.. what if we just join the strips together.. and make a raft?

 

Rafts are a different animal but they work a bit like concrete beams with steel to carry the tension loads, the concrete takes the compression. If you have plenty depth to play with, often occurs with rafts compared with say a concrete beam holding up a floor where depth is a factor, then you explore different thickness' of slab. This is partly driven by the soil and if you have insulated under the slab. The secret here is to find a slab thickness that uses the least steel and just as importantly to make the steel common sizes, easy to source, easily read drawings and quick to fix. This widens the field when finding a contractor.. go for the simple stupid! It's easier to pour an extra 50 - 75mm of concrete into a found than make / use more steel with complex bends and laps say.

 

Putting insulation under a slab is not difficult provided you keep it simple.

 

I would try if you can to get to grips with this. Then if you can't make it work start to look at other options. What you do learn will serve you well even if you find the strips and rafts are not suitable.

 

 

 

 

 

 

 

 

 

 

 

 

 

[Zak S]

@Gus Potter really appreciate you providing such a detailed analysis and some great tips. I am very keen to learn however, the requirement for experts would apply at the end. Please see below the picture of boreholes to 5m. They went down to 15m and found more solid stuff at 12m. The clay was very soft and I could easily press. High Water table at 1m. The area used to be a coppice and there is an area wide TPO hence lots of tree but most at least 10m away from the house.

[SimonD]

11 hours ago, Zak S said:

but it wont pass the building regs these days and we want to build on top

 

I think as @epsilonGreedy and @Gus Potter have said, don't count your chickens yet. I'm building on clay and we decided not to go for complete new build but use a large proportion of the existing bungalow for several reasons - time will tell whether this was a mistake financially. The biggest pain has been dealing with existing structure that isn't square, true or plumb, so I've been doing a lot of swearing. Our bungalow was built in the late 1920s  and the existing foundations are in some cases as little as 150mm deep and about 450mm wide in some parts they are as deep as 1m. We've added a complete story plus infill extension and the total weight of the new build is less than that of the original roof that held concrete tiles. Structural engineer dealt with some point loads using pad foundations where necessary.

 

I suggest you first look at designing what you want and then consider which way is best to achive that, extending existing or knock down and rebuild.

Edited December 15, 2021 by SimonD

[Gus Potter]

Hi Zak S.

 

Yes that looks pretty soft right enough at certain depths. Looking at the cores the ones to the right seem to be from the surface, the left ones in the photo from greater depth? The shallower cores look like there is maybe a stiffer crust with soft clay below and that the bungalow founds are using this crust to spread the load out before it reaches the softer clays...

 

The folk that designed the founds for the bungalow may have taken advantage of this crust. I would still expore the raft first. If the water table is high and not likely to drop then the soft clay can't shrink as it will always be submerged. It also can't go anywhere.. it is confined at depth. Once you go down a bit the pressures in the ground from a raft are often found to be very modest so even a soft clay can carry a surprising amount of load with acceptable settlement.

 

 

[Zak S]

3 minutes ago, SimonD said:

 

If you dig trial ditches around your existing foundations, make sure you get your structural engineer to look at the soil immediately as if you leave them open for any length of the time the soil will change consistency and may negatively affect foundation design. Also make sure you dig in several places as clay can and does often vary across the site.

Hi. Very interesting to see you doing something which I have moved away on the grounds that it was not worth the pain/savings etc. I would definitely not want to waste the existing c200 sqm. I had asked the soil investigation team to check the existing foundation so on the front its 1m and back 400mm. I did not show it to structural engineer as I currently dont have as I have just g appointed a planning/design architect and structural engineer would be down the line but I suppose once I get the GI report at that stage I can speak with SE to better inform the way forward. 

 

In remodel, I would be planning for an extension at the back and extension at the front and one side so not much will be left in terms of wall. But remodel is definitely on the cards if the rebuild costing does not work.