You can do these yourself
No qualification is attached to any of them. They are paper exercises with a published method, and if you can read a figure off a specification sheet you can produce one.
- The Part G water calculation every new dwelling needs — 125 litres per person per day, or 110 where planning says so. Ten minutes with your sanitaryware list. What Building Control actually asks for.
- U-values for your walls, roof and floor — free, including the PDF, so you can check whether 100 mm of PIR really does it before the order goes in.
- Soakaway sizing to BRE Digest 365 from your percolation test.
These ones you cannot
A SAP calculation has to be done by an accredited assessor and lodged under their registration. Your dwelling needs one at design stage, before you start, and a second one as built — plus the BREL report with its photographs, and the EPC on completion. Building Control will not sign the house off without them, and there is no free calculator for it anywhere.
The air tightness test is a physical test with a blower door, so that is a visit rather than a calculation.
Roughly when each one lands
Before you start: design-stage SAP. This is the one worth doing properly, because it is the only point where changing the answer is free.
Before the bathroom order: the Part G water calculation. A normal specification passes by about a litre, so a bigger bath or a pumped shower can quietly turn a compliant scheme into a non-compliant one after the tiles are on.
At completion: air tightness test, as-built SAP, the photographic evidence, the EPC, and the Part G notice — which regulation 37 says must reach the building control authority no later than five days after the work is finished.
Common questions
Do I need to be qualified to do the water calculation?
No. There is no accreditation attached to it the way there is to SAP. It is a paper exercise based on the fittings you are installing, and anyone who can read a flow rate off a specification sheet can produce a compliant one.
Why can I not just do the SAP myself too?
Because it is lodged, not just calculated. An accredited assessor registers the assessment and the EPC under their own accreditation, which is what makes it acceptable to Building Control. The software is not the barrier; the accreditation is.
Is it cheaper to bundle everything with one firm?
Usually, and more importantly it is less likely to go wrong. The failure mode on a self-build is not one bad report — it is two consultants assuming different things about the same wall.
Not quite you?
- Architects & technologists — Settle the fabric at design stage, then hand us the compliance.
- Developers & housebuilders — Multi-plot compliance, priced by dwelling type rather than by plot.
- Assessors & consultants — The calculators, the working shown, and your name on the report.