When an extension needs a SAP calculation
Most extensions never do. An extension to an existing dwelling complies with Part L by meeting the limiting U-values for new elements — walls 0.18, floors 0.18, roof 0.15, windows and glazed doors 1.4, other doors 1.4, rooflights 2.2 W/m²K — and by keeping its openings within the 25% rule: the total area of windows, roof windows, rooflights and doors should not exceed 25% of the extension’s floor area.
There is a useful allowance. Any windows or doors in the existing house that the extension removes or covers over stop losing heat to the outside, so their area is added to the 25%. A rear extension that swallows a set of patio doors and two windows can have noticeably more glass than the bare percentage suggests.
Interactive check
Is your extension over-glazed?
Three numbers off the drawings. You get the allowance, how far inside or over it you are, and which route that points to — before any glazing is ordered.
Go over the limit — the glazed rear wall, the roof lantern over the kitchen, the bifolds across the whole elevation — and there are two ways to show “reasonable provision”.
The two routes when you exceed 25%
Route one — area-weighted U-value. Improve the extension’s other elements so that its area-weighted U-value is no worse than that of a compliant extension with 25% glazing. More roof insulation and a better window specification are the usual levers. It keeps the calculation to the extension alone, and it runs out of road quickly when the glazing goes well past the limit. You can run it yourself, free, in GlazeMonkey: it builds the notional extension for you, shows the two tables side by side, tells you what the glazing would need to be if you are short, and exports a Building Control PDF for £35 +VAT.
Route two — the whole-dwelling SAP. Two calculations of the whole house with its extension. The notional version has the extension’s glazing capped at the 25% allowance and its elements at the standard values, and sets the target. The proposed version is the extension as you actually want it. If the proposed dwelling performs no worse than the notional one, it complies. This is the route Building Control means when it asks for “SAP calculations” on an extension, and it is the one with headroom: because the whole house is in both calculations, improvements to the existing dwelling — replacement windows, loft insulation topped up, a new boiler or heat pump — offset the extra glazing. In practice a well-specified extension often passes without touching the existing house at all, on solar gain and a decent roof.
If you want the whole energy side of an extension in one place before deciding which route you are on, the building regulations for an extension guide covers the allowance, the U-values and all three routes.
None of the new-build machinery applies to either route. There is no target emission rate for an extension, no design-stage or as-built notice, no BREL report and no photographs — those follow the erection of a building. You show the calculation, Building Control checks it.
Conservatories and porches
A conservatory or porch is exempt from Part L’s energy efficiency requirements only if every one of these holds: it is at ground level; its floor area is no more than 30 m²; it is thermally separated from the house by the existing walls, doors and windows, kept or replaced with ones that meet the energy efficiency requirements; and the house’s heating system is not extended into it. Take the doors out to make it part of the kitchen, or run a radiator off the boiler, and it becomes an ordinary extension: the U-value standards and the 25% rule apply, and for a conservatory that is almost entirely glass, that means one of the two routes above.
What we produce
For route two: the notional and proposed calculations for the whole dwelling, in a report Building Control can follow, with the pass shown and, where the first run failed, the changes that got it through and why. For route one: the area-weighted calculation. For either, before anything is ordered: a plain answer to “does it pass as drawn, and if not, what is the cheapest fix?”
What we need
Existing and proposed plans and elevations, and a basic specification — the glazing you have in mind, the roof build-up, what the existing house has for heating and insulation. It is a desktop study — no site visit — and turnaround is fast. Send the drawings before the glazing is ordered; the fix is cheap on paper and expensive once the frames have arrived. Get a quote or call 01202 623236.
Extending and converting at the same time? See SAP for conversions. Want to check a wall or roof build-up against the limiting values yourself first? U-Monkey does it free.
Common questions
Do I need a SAP calculation for an extension?
Usually not. An extension complies on its element U-values — walls 0.18, floors 0.18, roof 0.15, windows and doors 1.4, rooflights 2.2 W/m²K — provided the total area of windows, doors, roof windows and rooflights does not exceed 25% of the extension's floor area, plus the area of any existing windows or doors the extension covers up or removes. Go over that and you need one of two things: an area-weighted U-value calculation showing the extension performs no worse than a compliant one, or a whole-dwelling SAP calculation. A SAP is only required if you choose the second route.
What is the 25% glazing rule?
Approved Document L's limit on openings in an extension to an existing dwelling: the area of windows, roof windows, rooflights and doors should not exceed 25% of the floor area of the extension. You can add to that 25% the area of any windows or doors in the existing house that the extension removes or covers over, because those openings no longer lose heat to the outside.
What counts as glazing?
Every opening in the extension's external envelope: windows, external doors including fully glazed bifolds and sliders, roof windows, rooflights, roof lanterns and glazed roofs. It is measured as opening area — the whole frame, not just the glass — against the internal floor area of the extension.
How does the whole-dwelling SAP route work?
Two calculations of the whole house plus extension. A notional version, with the extension's glazing capped at the 25% allowance and the elements at the standard values, sets the target; the proposed version uses the extension as actually designed. If the proposed dwelling performs no worse than the notional one, it complies. Because the whole house is in both calculations, improvements to the existing dwelling — new windows, loft insulation topped up, a new boiler — offset the extra glazing.
Do I need the design-stage and as-built notices for an extension?
No. The design-stage and as-built notification duties, the target emission rate and the BREL report all apply where a building is erected. An extension is not a new building, so none of that machinery applies — you show compliance through the element standards or one of the two over-glazing routes, and Building Control checks it in the usual way.
Is my conservatory exempt from Part L?
Only if it meets all of the conditions: at ground level, no more than 30 m² of floor area, glazed to the safety-glazing requirements, thermally separated from the house by the existing walls, doors and windows (kept, or replaced with compliant ones), and with the house's heating system not extended into it. Break any one — take the doors out, run a radiator off the boiler — and it is an ordinary extension subject to the full Part L rules, including the 25% limit.
How quickly can you tell me if it passes?
From existing and proposed plans and a basic specification we can usually tell you the same day whether the extension passes as drawn, and if not, which of the fixes — better glazing, more roof insulation, something on the existing house — gets it through for the least money. It is a desktop calculation; there is no site visit.
How much does a SAP calculation for an extension cost?
It is quoted from the drawings, and it is a desktop job with no site visit, so it is one of the smaller items on an extension. The price depends on the route: the area-weighted U-value check is quick; the whole-dwelling SAP means modelling the existing house as well as the extension, so it takes longer and costs more — but it is the route that passes heavily glazed designs. Send the plans and we will tell you which one you need and what it costs, usually the same day.
What do you need to run the calculation?
Existing and proposed plans and elevations, a section through the extension, the glazing schedule with the frame type and U-values if they have been chosen, the wall, floor and roof build-ups, and — for the whole-dwelling route — what the existing house has for heating, hot water, insulation and windows. Photographs of the existing house help where the construction is not obvious from the drawings.