Do You Need a SAP Calculation for an Extension?
Most extensions don't need one. Go over the 25% glazing limit and you need another way to show Part L compliance. Here is how it works.
Do You Need a SAP Calculation for an Extension?
Most extensions don't. If the extension meets the standard U-values and keeps its windows, roof windows and doors within 25% of its floor area, it complies without a SAP. Go over that limit, which most kitchen extensions with bi-folds and rooflights do, and you have to show compliance another way. A whole-house SAP calculation is the most flexible way to do it.
The standard route: U-values and the 25% rule
Approved Document L Volume 1 sets limiting U-values for new elements in an existing home. For an extension these are:
- Walls: 0.18 W/m²K
- Floors: 0.18 W/m²K
- Roofs: 0.15 W/m²K
- Windows: 1.4 W/m²K, or Window Energy Rating band B
- Rooflights: 2.2 W/m²K
- Doors: 1.4 W/m²K, or Doorset Energy Rating band B (band C where more than 60% of the door is glazed)
The floor U-value can be worked out using the exposed perimeter and floor area of the whole enlarged house, not just the extension. That often helps.
On top of the U-values, the total area of windows, roof windows and doors in the extension should not be more than 25% of the extension's floor area. You can add the area of any existing windows and doors that the extension removes or covers. A 30 m² extension allows 7.5 m² of openings. If it covers an existing 4 m² patio door and window, the allowance becomes 11.5 m².
When you go over 25%
Large sliding doors, a glazed gable and a couple of roof windows soon take a kitchen extension past 25%. That doesn't mean the design fails. It means you have to show that the extra heat loss is made up somewhere else. There are two ways to do that.
Area-weighted U-value. We compare the extension as designed with an extension of the same size and shape built to the standard U-values and the 25% limit. If the area-weighted U-value of yours is no worse, it complies. This works when the extra glazing is modest and better walls, roof or glass can make up for it.
Whole-house SAP calculation. We model the existing house with the proposed extension, then the same house with a notional extension that meets the standard limits. If the proposed version performs no worse, it complies. Because it looks at the whole house, it can take account of upgrades to the existing home, such as a new boiler, loft insulation or replacement windows. Those upgrades are building work too, so they go on the building control application. This route gives you the most design freedom.
When the energy rules don't apply
A conservatory or porch is exempt from the energy efficiency requirements if all of these apply:
- It is at ground level and no more than 30 m².
- Its glazing meets Part K.
- The wall, door or window between it and the house is kept or replaced.
- The house's heating system does not extend into it.
Run a radiator into it, or take out the separating wall or doors, and it is treated as an extension that has to comply.
Part O and extensions
Part O overheating applies to new residential buildings. An extension to an existing house does not need a Part O assessment.
The new Part L from March 2027
The 2026 edition of Approved Document L Volume 1 takes effect on 24 March 2027 (24 September 2027 for higher-risk building work). The limiting U-values for new elements in existing homes are the same as today. Work notified to building control before 24 March 2027 and started before 24 March 2028 can stay on the current rules. The details are in Circular Letter 01/2026.
What we need from you
- Floor plans, elevations and sections of the existing house and the extension.
- Sizes of every window, door, roof window and rooflight, including the existing openings the extension removes or covers.
- Wall, roof and floor build-ups, or the U-values you are aiming for.
- The glazing specification.
- The existing house's heating system and any upgrades you plan.
Price and turnaround
SAP calculations start from £100+VAT. Send the drawings and we'll confirm the price for your extension. Turnaround is 3 working days once we have everything. With 17 years and more than 4,000 SAP calculations behind us, we'll tell you early if the glazing or the spec needs to change, and what the cheapest fix is.
Questions we get asked
Do I need a SAP calculation for a single-storey rear extension? Only if it goes over the 25% opening limit or can't meet the standard U-values. If it stays within 25% and meets the U-values, the standard limits are enough.
Do roof windows count towards the 25%? Yes. The limit covers windows, roof windows and doors, so roof windows and glazed doors count along with the windows.
Can an extension SAP fail? Yes. If it does, we show you what to change: better glazing, more insulation, less glass, or an upgrade to the existing house. We can't promise a pass, but we will find the most affordable change that gets you there.
Is my conservatory exempt? If it is at ground level, no more than 30 m², separated from the house and not heated by the house's system, yes. If not, it is treated as an extension.
More: SAP calculations and EPCs, how much SAP calculations cost, SAP or SBEM, energy assessments for retrofit projects and the latest industry updates.
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