Your streamlined solution for Part L Extension Compliance, ensuring your building projects meet regulatory standards with automated Average U-value calculations and clear compliance reports.

SAPPER is a free AI-powered SAP (Standard Assessment Procedure) compliance calculator for UK Building Regulations Part L. It automates data extraction from architectural PDF drawings, calculates thermal bridges, checks Part L compliance using Average U-value or Whole House methods, and generates professional compliance reports. Built by CoreLogic Ai Ltd.

What are the Part L 2021 U-value limits for extensions?

Part L 2021 (England, from June 2022) extension limiting U-values: External walls 0.18 W/m²K, Roofs 0.15 W/m²K, Floors 0.18 W/m²K, Windows 1.4 W/m²K, Doors 1.4 W/m²K, Rooflights 2.2 W/m²K. These are used as notional values in the Average U-value compliance calculation.

How does AI extract SAP data from drawings?

SAPPER uses a multi-pass AI extraction system: Pass 1 classifies drawing types (plans, elevations, sections) and detects scales. Pass 2 extracts dimensional data (floor areas, perimeters, storey heights) from annotations. Pass 3 analyses every elevation for windows, doors, rooflights with orientation detection. Results are validated with sanity checks and achieve 85-95% accuracy.

What are thermal bridge psi-values?

Psi-values (ψ-values) measure additional heat loss at building junctions in W/mK (Watts per metre Kelvin). Common UK junctions include E2 (lintels, accredited ψ=0.20), E3 (sills, ψ=0.04), E4 (jambs, ψ=0.04), E5 (ground floor, ψ=0.12), E10/E11 (eaves, ψ=0.04-0.06). RCD (Recognised Construction Details) values are 20-40% lower for brick/block masonry constructions.

What is the condensation risk f-factor?

The f-factor (fRsi) is the surface temperature factor used to assess condensation and mould growth risk at thermal bridges per BR 497. f ≥ 0.75 = low risk (acceptable), f 0.65-0.75 = medium risk (review needed), f < 0.65 = high risk (mould growth likely, design modification required). Calculated as fRsi = (Tsi - Te) / (Ti - Te).

Is SAPPER free to use?

Yes. SAPPER is completely free with no feature limitations, no credit card required, and no hidden costs. Part L calculator, AI extraction, thermal bridging, room in roof calculator, and PDF reports are all included at no charge.

What does overglazed mean for an extension?

It means your extension has more glass, in proportion to its walls or floor, than the rules expect. There isn't one single legal definition — it's worked out two ways: how much of the extension's outside walls is glass, and how much glass there is compared with the floor area. Cross either line and it changes how your extension is classified and assessed.

What is the 25% rule?

It's about energy. Add up every square metre of glass — windows, doors, bifold doors and roof windows — and compare it with the extension's floor area. Stay at or below a quarter and the straightforward average U-value approach can be used for the extension. Go above a quarter and a whole-dwelling calculation is normally needed instead. The same quarter-of-floor-area figure appears in SAP, the government's energy calculation, as a fixed glazing allowance: once your actual glazing goes over it, a fixed allowance of that size is used in the calculation rather than your real glazed area.

Do bifold doors count as glazing?

Yes. Every square metre of a glazed bifold, patio door or French door counts in both tests. A wide bifold set is one of the most common reasons an extension tips over the 25% line, because a single 4 m by 2.1 m opening is over eight square metres of glass on its own.

Do skylights and roof windows count?

They count towards the 25% floor area test, because they're glass and they add heat loss. They don't count towards the 50% wall test, because that one only looks at the walls. Add them as skylights in the calculator and the difference is handled for you.

Does an overglazed extension need planning permission?

Overglazing doesn't create a planning requirement on its own — planning permission and the glazing tests are separate questions. What overglazing changes is how the extension is classified, and that classification feeds into both the planning rules and the building regulations. Many extensions are covered by permitted development; if yours isn't, or if it's being treated as a conservatory, take advice on both early rather than at the end.

Can I fix an overglazed extension?

Usually, yes, and often without changing the look. You can reduce the glass area, add solid wall such as a brick pier to bring the wall percentage down, upgrade to better-performing glazing, or run the whole-dwelling calculation instead of the simplified route. The cheapest fix is almost always at design stage — nudging the glass area under the line before anything is built.

Is this calculator accurate enough for building control?

It uses the same thresholds that building control and SAP assessors work to, so it's a reliable early check. But it's a guide, not a formal assessment: it can't see your drawings, your wall build-up or your heating system. Use it to find a problem early, then have the final figures confirmed by a registered SAP assessor.

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Overglazed Extension Calculator

Too much glass can change how your extension is classified and how it has to be assessed for UK planning and building regulations. Put in your measurements, list your windows and doors, and find out straight away whether your extension is overglazed — in plain English, with no jargon.

Check your extension

A tape measure and two minutes is all you need. Your results update as you type.

Step 1 · Your extension

Floor area (m²)

Length × width of the new room or rooms.

Outside wall area (m²)

Measure around the outside, then × the wall height. Include the parts where the windows are.

Step 2 · Your glass

Add every window, door, bifold and roof window. Leave a row blank if you don't need it.

Glass 1

Width (m)

Height (m)

0.00 m²

Glass 2

Width (m)

Height (m)

0.00 m²

In the walls: 0.00 m²In the roof: 0.00 m²Total glass: 0.00 m²

Your result

The 50% rule — how your extension is classified

Is your extension really a conservatory?

Add the outside wall area of your extension to run this check.

The 25% rule — building regulations

How your glass affects your energy assessment

Add the floor area of your extension to run this check.

This calculator is free guidance, not a formal assessment. Your building control officer or SAP assessor makes the final call.

What counts as an overglazed extension?

Overglazing isn't a single number written into the regulations — it's the point at which the amount of glass in your extension starts to change how it's treated. There are two separate tests, and an extension can fail one, both, or neither.

The first looks at the walls: how much of the extension's outside wall surface is glass. The second looks at the whole extension: how much glass there is compared with the floor area. The first affects how your extension is classified — as a normal extension or as a conservatory. The second affects which energy calculation your extension needs.

The two tests that decide it

The 50% rule: is it an extension or a conservatory?

If more than half the area of the extension's outside walls is made up of windows, doors or similar glazing, it's generally treated as a conservatory. This is why a rear extension with a wall of glass and very little brick tends to get called a conservatory, while the same footprint with more solid wall doesn't.

Being treated as a conservatory isn't automatically a problem, but it changes the rules. Conservatories can be exempt from the energy efficiency requirements of the building regulations, but only where all of the following apply: it's at ground level; it's separated from the house by walls, doors or windows that meet the thermal requirements; its heating and cooling are independent of the house, or it has none; and its floor area is no more than 30 square metres. Miss any one of those conditions and the extension is treated as a normal extension, with the full requirements applying.

The 25% rule: glass against floor area

Add up every square metre of glass — windows, doors, bifold doors and roof windows — and compare it with the extension's floor area. Stay at or below a quarter and the straightforward route can be used: the average U-value approach, which treats the extension's walls, roof, floor, windows and doors as a package and checks the overall figure.

Go above a quarter and that simplified route is no longer available. The extension has to be assessed as part of the whole house instead, comparing your home's energy performance before and after the extension. That's a bigger piece of work, and it's much more sensitive to the rest of the house — an old boiler or uninsulated walls elsewhere can drag your extension's result down.

The same quarter-of-floor-area figure appears in SAP, the government's energy calculation, as a fixed glazing allowance. Once your actual glazing goes over it, a fixed allowance of that size is used in the calculation instead of your real glazed area.

Why glass is the weak point

A typical new extension wall is built to around 0.18 W/m²K. A good double-glazed window is around 1.4, and even a top-performing triple-glazed unit is only around 0.8. In plain terms, every square metre of glass loses roughly four to eight times as much heat as the wall it replaces.

That's why the rules care about the ratio rather than the total. A large extension can carry a lot of glass and still be fine, because there's plenty of wall and floor to balance it against. A small extension with one wide bifold can be over the line immediately — a single 4 m by 2.1 m opening is over eight square metres of glass on its own.

Glass also brings solar gain. A lot of south- or west-facing glazing makes a room uncomfortable in summer, and overheating is now a building regulations concern in its own right.

How to check your extension in under a minute

1
Measure your floor area. Take the length and width of the new room or rooms and multiply them together. A 5 m by 4 m extension is 20 square metres.
2
Measure your outside wall area. Measure all the way around the outside of the extension, then multiply by the wall height. Use the full wall area, including the parts where the windows and doors are.
3
List every bit of glass. Add each window, door, bifold and roof window with its width and height. Mark roof windows as skylights so they're counted in the right test.
4
Read your two results. The first tells you how your extension may be classified. The second tells you which energy calculation it will need.
5
Deal with anything that's over. If either check is over the line, reduce the glass, add more solid wall, or improve the glazing specification before you submit.

What to do if your extension is over the line

Reduce the glass area: Taking a window down a size, or swapping a glazed gable for solid wall, moves both percentages at once.
Add solid wall: A brick pier or a section of solid wall brings the wall glazing percentage down without shrinking the extension.
Improve the glazing: Better-performing units don't change the ratios, but they help the energy result once you're over the 25% line.
Run the whole-house calculation: If you're over 25% and want to keep the design, that's normally the route. Get an assessor involved early.
Sort it at design stage: Almost every fix is cheaper, and some are free, before anything is built.

Overglazed extension questions

Straight answers on the glazing rules, in plain English.

Need the full calculation?

The checker tells you where you stand. The full assessment works out the average U-value, the thermal bridges and the compliance result, then produces a report you can hand to building control — from £45, paid once.

Run the full Part L assessment