When should design-stage SAP be done?

Before the specification is locked. Run early, the calculation shows which compliance route is cheapest for each plot type; run late, it only confirms decisions you can no longer change cheaply.

Why do as-built results differ from design stage?

Because as-built calculations use what was actually installed plus the tested airtightness figure. Product substitutions and a worse-than-assumed air test both move the result.

Do all plots need an air pressure test?

Testing requirements depend on the site and dwelling types, with sample testing applied in defined circumstances. The tested figures feed the as-built calculations for the plots they represent.

Can plots be sold before on-completion EPCs are lodged?

No. A valid EPC is required when a plot is marketed for sale or let, and it comes out of the as-built work, so incomplete as-built evidence delays sales directly.

Who advises on the cheapest way to comply?

Awais Sarwar handles Part L strategy on our developments directly, so the person advising at design stage is accountable for the as-built numbers.

Guide · guides

Part L and SAP for developers: what you need at each stage

Part L problems are almost always sequencing problems. Design-stage SAP done late, or as-built evidence collected after the fact, is what turns compliance into a delay.

11 min readBy Nabhan Islam Reviewed by Awais Sarwar, MSc Updated 9 Sept 2026

The short answer

A development needs design-stage SAP calculations before construction, showing each plot can comply with Part L, and as-built calculations afterwards using the specification actually installed, supported by air pressure test results and commissioning evidence. On-completion EPCs are produced from the as-built calculations and are needed before plots can be sold or let. The practical rule is that design-stage SAP should inform the specification rather than confirm it: run it early, and fabric, glazing, heating and ventilation choices can be adjusted on paper instead of on site.

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Design stage output

SAP compliance report per plot

before construction, ideally before specification is locked

As-built output

as-built SAP plus on-completion EPC

based on what was actually installed

Evidence needed

air test, commissioning, product data

collected during build, not reconstructed later

Sale/let requirement

on-completion EPC

required before plots are marketed

Design stage: use SAP to make decisions

Design-stage SAP calculations model each plot against Part L targets using the intended fabric, glazing, heating, ventilation and any renewables. The mistake most developments make is treating this as a compliance certificate to obtain once drawings are frozen. Run early, the same calculation is a design tool: it shows whether tightening the fabric, changing a glazing ratio, upgrading a heating system or adding photovoltaics is the cheapest route to compliance on that specific plot type. Changes made at this stage cost drawing time; the same changes made after the frame is up cost money and programme.

Model each plot type rather than assuming one calculation covers the site
Compare compliance routes on cost, not just on whether they pass
Fix glazing ratios, fabric build-ups and services strategy while they are still on paper
Flag plots that only just comply — they are the ones that fail after a site substitution

Specification decisions that move the numbers

A handful of inputs dominate the result: wall, roof and floor U-values, window and door performance including frame factors, thermal bridging detailing, airtightness target, heating system efficiency and controls, ventilation strategy, hot water provision and any on-site generation. Thermal bridging is the one most often left at default values, which is a quiet penalty: using accredited construction details where they genuinely apply can recover meaningful margin. Likewise an ambitious airtightness figure assumed at design stage becomes a liability if the build cannot achieve it, because the as-built calculation must use the tested figure.

U-values, glazing performance and thermal bridging are the biggest fabric levers
Heating efficiency, controls and hot water strategy dominate the services side
Assume only the airtightness figure the build can realistically achieve and test
Renewables should be a considered choice, not a rescue measure late in the programme

The buildings behind this advice

As-built: what was installed, not what was drawn

As-built SAP calculations rerun the model using the specification actually installed, backed by evidence: product data sheets for insulation, windows and doors, heating and ventilation commissioning records, and the air pressure test results for the tested plots. Where a substitution happened on site — a different window supplier, an alternative insulation board, a changed boiler or heat pump model — the as-built figure changes with it, and a plot that passed at design stage can fail. The fix is dull but reliable: collect evidence as work proceeds and flag substitutions to whoever holds the SAP model before they are installed across the whole site rather than one plot.

Keep product data and commissioning records as they are generated on site
Notify substitutions before they are repeated across every plot
Air pressure test results feed directly into the as-built calculation
Expect plot-by-plot variation and check the marginal plots first

On-completion EPCs and the sales programme

On-completion EPCs are produced from the as-built calculations, so they cannot be issued until as-built data is complete. This is where compliance sequencing meets commercial reality: plots cannot be marketed for sale or let without a valid certificate, so incomplete as-built evidence directly delays exchange. Where a site completes in phases, it is far better to close out as-built work phase by phase and lodge certificates as plots finish, rather than batching everything to the end of the programme and discovering a missing air test on the day sales need paperwork.

On-completion EPCs depend on as-built calculations being finished
Lodge certificates phase by phase as plots complete
Missing air tests and commissioning records are the usual cause of last-minute delays
Keep a plot tracker showing design-stage status, as-built status and lodged certificate

How we run it on a development

On a multi-plot site we work from the plot schedule and drawings: design-stage calculations by plot type with a written note on which compliance route is cheapest, a specification watch-list of the inputs that must not change without telling us, then as-built calculations and on-completion EPCs closed out phase by phase. Awais Sarwar handles the compliance strategy directly, so the person advising on specification is the person accountable for the numbers at as-built stage. Where the same site also needs commercial assessment, fire risk or electrical certification for communal parts, those run under the same instruction to avoid three separate mobilisations and three separate points of contact.

Design-stage calculations by plot type, with a costed view of compliance routes
A watch-list of inputs that must not be substituted silently
As-built calculations and on-completion EPCs closed out per phase
One instruction covering commercial, fire and electrical elements where the site needs them

Written by

Nabhan Islam

Head of Marketing & Commercial Lead

Reviewed by

Awais Sarwar, MSc

Accredited Energy Assessor — Accredited domestic and commercial energy assessor, listed on the EPC Register

11 min readLast reviewed 9 September 2026Facts verified 9 September 2026 Checked quarterly

Sources

The Energy Performance of Buildings (England and Wales) Regulations 2012 — legislation.gov.uk
Find an energy certificate — GOV.UK

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