SAP Excess Glazing Calculation for Extension

FaceHead

Hi Folks,

I'm re-building the side return extension on a terraced house. By re-building (long story), currently existent is something which is half a conservatory (translucent roof) and half an extension (integrated heating, no doors into it). To me it doesn't seem compliant with any regs (not noted by my surveyor or solicitor before purchase), but it's too old to be vulnerable to any action.  It's leaking, and generally rubbish, so I plan to demolish it and rebuild allowing me to remove some internal walls and install roof windows and a bifold door. The 'extension' will be fractionally increased in size.

I've had planning approved, had a structural design done and agreed a price with a builder.  To get it through building control, am I now right in thinking I need an excess glazing calculation? The glazing will almost certainly be more than 25% of the new floor area, although that depends on the definition of 'new floor area' (the area is actually barely changing), and what glazing is included (the majority of the new glazing is the new bifold door, which is largely in the original house, and only partly in the rebuilt area).

Assuming the next step is to get the excess glazing calculation done and determine that I do indeed need to insulate the loft to pass building control, can someone recommend a company to do the SAP calculation, or where to find one please. My structural engineer has offered to do (= subcontract) this for £275+VAT, which seems steep.  

Any general thoughts/input welcome. Perhaps at this stage I should engage a building inspector and they'll tell me to go away and get the SAP calculation done?

TIA.

Doozergirl

The calculations only take a few days to come back, so just do it if you are asked to.   We have only been asked once!  

I paid £185 plus VAT to a company I found online. 

FaceHead

Doozergirl said:

The calculations only take a few days to come back, so just do it if you are asked to.   We have only been asked once!  

I paid £185 plus VAT to a company I found online. 

Thanks for your reply. By 'asked to' do you mean asked to by the building inspector? So my next step is to choose and building inspector and provide everything I have to them, to be told what else I need? 

DevilDamo

Only provide BC with a SAP calculation if asked to. They’ll soon check whether or not the proposals meet Part L.

You’ve worked out 25% of the ‘internal’ floor area, which is your allowable limit for any proposed windows, doors and rooflights. Also remember the area of the existing openings being covered by the extension will also increase your percentage. If after working that out you find you’re still over the threshold, you will more than likely require a SAP calculation.

ComicGeek

As long as the glazing isn't mainly north facing, the SAP calculation will just be a paperwork exercise as the beneficial solar heat gains will outweigh any additional heat loss. Unfortunately the process normally involves a site survey, measuring all floor plans, measuring all windows and doors, counting light bulbs, so ends up being much more time intensive than calculations for a new build house. With the hundreds I've completed over the years, I've never had a situation where we've had to upgrade anything in the existing house to offset.

We charge around £350 + vat as can take at least half a day to complete even for a small extension.

The other way is to undertake an area weighted U-value calc, but this typically requires a much more onerous level of insulation and isn't normally worth spending any time/money looking at.