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What is going on top of the pour, we had 3 at some points like you, but our pour is 200 with insulation and screed on top. So, if the concrete is a bit thin at points then it will be well covered.

I did one in Gravenhill and the B&B had regular blocks, and then we put 200mm of PIR and 100mm of concrete; reinforced by 2 layers of mesh. The design and storey rods help you to calculate how low the B&B needs to be set down to allow the extra insulation. 75mm of EPS is pretty crap.

Depending on size on patio, you could throw down broken glass(smash up alot of bottles) under the pedestals/pavers. Rats won't bother with it. Go to local pub or often times bottle banks are full and people just leave bottles beside em, and it's free.

Afternoon all I have just had an inspection visit for my timber frame rear extension. It is a single storey 6x2 timber frame, 400 centres, 11mm OSB on the outside, breather membrane and is to be finished with vertical thermo pine, on batten / counter batten. The BCO has advised I need to put intumescent cavity closers in at the top / bottom and around the windows and doors, which is not something I thought was required for ventilated cladding; I've just got mesh top and bottom. Windows / doors have a cladding cheek to finish, so are closed already. Can anyone on here enlighten me? John

I've never had a design or installation queried for foul or surface water. I'd have to read the docs again, but I think the proposal of a Chartered Engineer working to the principles of these docs and referring to site specifics will be accepted. Eg. Scotland allows 20% reduction when using a treatment tank. England does not, but it is in the tech papers so apply it if it's necessary. Basically if you use a good treatment tank it's going to be ok, they want it to be OK, and so come up with a justification.

Anyone with any success producing irregular shaped fields for awkward sites?

This document titled 'Advice for local authorities on non-mains drainage from non-major development' is found on the planning portal. It's purpose is to assist BCO's in deciding whether a scheme needs EA involvement or not. It's flow chart references BS 6297 and the general binding rules (which itself references BS 6297) consistently for determining whether or not EA involvement is required - but not part H. EA_LPA_advice_non_major_dev_non_mains_drainage_2019.pdf

I've not tested this but I do wonder - if you presented the EA with a field design with 1 metre spacings between trenches (ie fully compliant with BS 6297), would it be approved as a bespoke solution (ie do this in response to a stubborn BCO)? A British water engineer I know often specifies 1 meter between trenches on tight sites. This part of the regs needs overhauling asap. There are tens of thousands of poorly functioning septic tanks across the country in gardens that have no hope of accommodating a compliant drainage field. All you have to do is go for a cycle around your local area in this weather to smell it!

Fair point and thanks for your helpful interpretation.

Its not that bold. A normal ground bearing floor is susceptible to heave, suspended one less so. I have trees very close to proposed house. Better to plan for the worst. I started the thread as I was interested in the difference between the cold bridges on the 2 different systems. I wasn't really looking for a discussion on whether to use ground bearing or suspended floor. This always happens on this forum. I can see why it has quietened down with ai.

Yes, this is how I set up check lists at work. One can usually find the person who has forgotten all about it.

What I have just posted (and which was commented on in a previous post) relates to competency; point 3) in the SAP guide which reads; "Using -values calculated by a person with suitable expertise and experience using the guidance set out in BR 497, and BRE IP 1/06. 84 SAP version 10.3 (13-01-2026)" I have really tried to get to the bottom of this. I was quoted thousands to get a report done so looked for alternatives and worried about this very point but after talking with my SAP assessor decided I could do this declaration myself. In getting to this place I trawled the internet and concluded the following: Under UK Building Regulations (SAP 10 / Part L), "competent person" has no statutory definition for thermal bridging. There is no state license, no mandatory qualification, and no legal registry required to submit a bespoke Psi-value. When someone signs the report, they are only signing off on three specific facts: Geometry & Materials: "I confirm this computer model actually matches Drawing No. XYZ and the real material specs for this project." Methodology: "I confirm the software executed the calculation using BR 497 and BS EN ISO 10211 standards." Condensation Check: "I confirm the junction was checked for mould/condensation risk fRsi." Why the SAP Assessor Cares A SAP assessor cannot just take a custom Psi-value written on a post-it note. If Building Control or their accreditation body (like Elmhurst or Stroma) audits their SAP file, the assessor must show a document that ties that number to the actual build. The signature simply provides accountability. It tells the auditor: "A real person verified the inputs and produced these results using software that has been verified against ISO 10211:2017 and complies with BR 497 requirements." The signature isn't a badge of professional status; it's a declaration of accuracy.

You should be able to take measurements internally/externally in the same fashion. That diagram is perfect. The internal measurements compared to external will be different and depends on finishes, they could be the same but if brick to brick externally, then I’d expect internal to be slightly smaller as the window measurement will be reduced by the plasterboard (if it is not sitting behind the ingo/check reveal). I doubt it is a check reveal if you’re in England.

A bold assertion without knowing the detail. Everything on its merits.

From SAP version 10.3 (13-01-2026), which is the definitive government document; There are several possibilities for specifying the thermal bridging in SAP: 1) Details from a government-approved source involving independent assessment of the construction method of junctions and provision of -values for that junction. 2) Details from a reputable non-government database containing independently assessed thermal junction details. 3) Using -values calculated by a person with suitable expertise and experience using the guidance set out in BR 497, and BRE IP 1/06. 84 SAP version 10.3 (13-01-2026) 4) If none of the above applies use equation (K2), with y = 0.20 W/m²K. Manufacturer data is not government approved or independently assessed. Table K1, which follows then provides the defaults if none of the above apply. The document states before the table: "The -values given in table K1 are conservative defaults, therefore it is recommended to derive values by numerical modelling, which will be more accurate." If you compare the table values with SAP10.2 you will see that they are not just conservative but punitive. SAP 10.3 Full Specification 2026-01-13.pdf

I read that as..." how much detail do we need to go into? For examples: A builder may have been asked to do this for the first time. A self builder may have been "sold" the idea at an exhibition, without having the pros and cons explained.* *there is a reason I see such systems at self build exhibitions but not at trade exhibitions. Not that it's bad, but that there are options and there is more to know.

Presumably you do have a Party Wall Agreement in place ? And you have ensured that the eaves and gutter will not project onto your land (which is illegal) and this is clearly defined in the drawings within your PWA. Presumably there are no windows in that wall and overlooking your property ? Nor vents or external pipes ?

Yes, I get it. Thanks for your alert. Choosing the foundations is one of the most important decisions in the project. The best one can hope for is to become an "intelligent customer". That's why I'm talking to people and asking questions on here.

The floor edges are dealt with as a linear thermal bridge separate from the insulation (& Uvalue) in the general floor area. The ground floor/wall junction is a difficult one when preventing/minimising the thermal bridge. This is more difficult with a suspended concrete slab that bears on inner masonry - 'overlapping' the cavity wall and floor insulation is one option. This is further complicated with beam and block. Relatively simple where the beams run parallel to the wall (2D thermal analysis) but probably requiring a 3D thermal analysis where the beams run into and bear on the wall. Again extending any cavity insulation to well below the floor insulation will help.

I still cannot find where SAP 10.3 does not allow manufacturer psi-values. Is there another guidance doc that you are referring to? Can you please post here?

That is only PART of the outcome. What happened next? Was it left but without BC signoff (would only be a problem if they sell) or did BC insist on remedial work?

Also certain trees mean deeper foundations which mean suspended floor is the better option incase of heave.

The system is designed by manufacturer. That bottom one is the finished article excluding dpm. Literally finished floor slab. On top of concrete beams which bear onto walls which touch the ground and are exposed to outside air. Of course you can potentially mitigate with insulation on top but who knows if then complies with bba cert. At least the top system has a nice chunky 75mm sheet on top of beams before concrete

There are two ways for me to understand your question. If it's a straight one, the straight answer is, no I don't do it for a day job/business and yes I'm a genuine self builder, whatever that is. But since I opened my post by explaining why I'm asking my question, the straight answer should already have been clear. So I'm wondering if your question was rhetorical and intended to demean and denigrate, which would be a pity. Thanks for your comment. Useful to get your opinion..

Not at all. There is a cold bridge but it is not completely cold if thought through with insulation above. A building is a cold bridge between indoors and outdoors... but it is relative. If near trees then the foundations will be deep and the trees won't affect the slab. The main downside of suspended floor is the essential draught under it. You lose the benefit of the natural insulation of the earth.