SAP calculations in Croydon

The Croydon house stock the calculation starts from

Around 60.2 per cent of Croydon households live in a house or bungalow, and 54.7 per cent own their home. Terraces, semis and detached houses each lose heat differently, so the calculation differs for each.

A terraced house averaged about £396,579 on average in the latest Land Registry figures, while a detached house reached £837,514. At the lower price the budget for glazing and heat recovery is tight, so we favour the elemental route with simple limits. On a detached plot the wider gap in value usually justifies a fuller SAP model that lets the designer keep a glazed rear elevation.

How the extension route works on a Croydon home

For new elements in an existing dwelling, Approved Document L gives this limit set: New elements in existing dwellings, limiting U-values W/(m²·K): roof 0.15, wall 0.18, floor 0.18, windows 1.4 (or WER Band B), doors 1.4. A masonry rear extension with a pitched roof can meet those figures without anything unusual, which is why many Croydon owners never need a calculation at all.

The glazing cap is where the design starts to push back. The guidance says: Windows, roof windows, rooflights and doors in an extension should not exceed 25% of the extension floor area plus the area of openings removed or covered. We count the openings on the drawing before the doors are chosen, and if the count fails we show you the alternative: Alternative routes: area-weighted U-value (para 10.9) or a SAP calculation against a notional extension (para 10.10).

That choice belongs in the drawings, not after them. Our rear extension drawings carry the wall and roof build-ups the assessor needs, so one set of sheets serves planning, building control and the energy figures.

Heritage at risk, scheduled sites and what we check first

Croydon lists 15 entries on the Heritage at Risk Register, and 8 scheduled monuments touch the borough. Neither rules out an extension on its own, but a property next to one can get a closer look at materials, and materials feed straight into the U-values.

Before any modelling we run the address against the designations, then agree the external finishes with you. Only then do we fix the construction build-up the assessor will use. Changing a facing brick to a render after the calculation is done is a small edit on site and a repeat of the work on paper.

New homes, flats and the changing energy edition

A new dwelling or a conversion into flats follows a different path from an extension. For new dwellings AD L 2026 names two methods: the Home Energy Model (HEM) and SAP; validity dates are in the Notice of Approval. The edition that applies depends on when plans are deposited, and the guidance sets the start: AD L 2026 takes effect 24 Mar 2027 (24 Sep 2027 for higher-risk building work).

Croydon builders working on infill plots should also note the overheating rule: Approved Document O (overheating) applies to new residential buildings. We pair the energy work with our new build drawings, because wall thickness and window size change the sections on both sets of sheets.

Your own drawings are the source of every figure, so we do not ask an assessor to measure from a sketch. The area of walls, roof and glazing is taken from the model, and any later change goes back through the same route.

What the assessor needs from you in Croydon

Send us the address, a photograph of the rear and the roof, and any decision notice already issued. From those we can say whether the job is a simple-limits case or a calculation. Owners often arrive with a quote from a builder that names the wall and window products, and we use that, because a supplier declared U-value is better evidence than a generic figure.

Heating is the item people forget. The assessor needs to know the boiler or heat pump, the cylinder size and the controls, and a late change of heating system can move the result noticeably. If you are undecided, we ask the assessor to model two options and show you the difference.

We return the calculation as a short report that building control can follow, with the assumptions listed beside the drawing references they come from. If the officer queries a figure, the page that explains it is already there.

Where the calculation sits in your Croydon package

A SAP result supports the technical drawings rather than replacing them. See our building regulations drawings in Croydon for the construction details, and architectural drawings in Croydon for the planning side and the local constraints in one place.

Pricing is a fixed quote after a free constraints check, VAT included, because a short extension check and a full new-dwelling assessment are very different jobs. Send the address and a short brief and we confirm the route first.