What you actually have to do, in order
Self-builders are usually managing everything at once, so the sequence matters more than the theory. The design-stage calculation happens with your building control application, before work starts. During the build you keep to the modelled specification, photograph the key details as they are covered, and keep the commissioning paperwork for the heating, hot water and ventilation. Near completion the air tightness test happens, we update the model with what was built, and the EPC is produced from it.
- Before work starts: design-stage SAP and BREL report to building control
- During the build: build to the specification and photograph the listed details
- Keep every commissioning certificate the installers give you
- Near completion: air tightness test
- At completion: as-built calculation, evidence pack, on-construction EPC
The mistakes that cost self-builders most
The two expensive ones are ordering fabric before the calculation exists, and photographing nothing. Ordering first means a failing design has to be fixed with services or renewables rather than insulation. Missing photographs cannot be recovered once the plasterboard is on, and the outcome then rests with your building control body. Both are free to avoid and expensive to fix.
Heating choice and the calculation
Your heating and hot water decision moves the result more than almost anything else, and it interacts with fabric: a well-insulated, airtight house with a heat pump behaves very differently in the calculation from the same house with a boiler. If the decision is still open, tell us and we will model the options together rather than one at a time, so you can see the trade-off before you commit.
About the 2026 changes
The 2026 Part L documents are published, with general commencement on 24 March 2027, and transitional arrangements can keep work under the current rules where the building control application was given before that date and work on the building starts before 24 March 2028. Which applies to your plot depends on your dates, so we check it at the start rather than assuming. Where your build sits close to the boundary, it is worth knowing early.
