U-value of an extension floor (and the whole-house perimeter rule)

A ground-bearing slab in a rear extension, and why the whole enlarged house’s perimeter and area can help.

A solid floor in a 4m × 5m rear extension — 100mm of PIR over the slab under a 65mm screed — has a U-value of about 0.18 W/m²K (0.177) on the extension’s own exposed perimeter and area (13m, 20m²), inside the 0.18 Approved Document L sets for a new floor in an existing home. Approved Document L also lets the floor of an extension be calculated with the exposed perimeter and floor area of the whole enlarged dwelling (Table 4.2, note 5): for a semi-detached house that takes the same floor to about 0.17, and lets 90mm of PIR pass (0.179) where on the extension alone it misses (0.192).

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The build-up (outside → inside)

Calculated to BS EN ISO 13370: the pre-filled link opens with the extension’s own exposed perimeter (13m: 4m + 5m + 4m) and area (20m²); the whole-house rows use 30m and 70m². It opens as a new element in an existing home, judged against Approved Document L’s 0.18 (2021 edition, Table 4.2).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
75mm PIR, extension alone≈ 0.22
75mm PIR, whole enlarged house≈ 0.20
90mm PIR, extension alone≈ 0.19Misses 0.18: 0.192
90mm PIR, whole enlarged house≈ 0.18Meets: 0.179
100mm PIR, extension alone≈ 0.18As pre-filled: 0.177
100mm PIR, whole enlarged house≈ 0.17
120mm PIR, extension alone≈ 0.15

Figures computed to BS EN ISO 13370 with BR 443 conventions, exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up and the footprint and depth shown: your products, thicknesses, perimeter, area and ground will move the number, and that's what the calculator is for.

What changes the number

A ground floor’s U-value depends on its shape as much as its build-up: BS EN ISO 13370 works from the exposed perimeter (P) and the area (A), and a small extension has a lot of edge for its size (P/A 0.65 here). Approved Document L lets you use the whole enlarged dwelling’s perimeter and area instead (note 5 to Table 4.2, and note 7 to Table 4.3 for a retained floor), which for this example — a 50m² semi with 22m of exposed wall, plus the extension — gives P/A 0.43 and lowers the U-value by about 0.01. Count only walls that bound the heated space: the wall between the extension and the house is not exposed perimeter, and nor is a party wall. Where the new floor has to line up with an existing one, note 4 to Table 4.2 allows a lesser standard if 0.18 would cause significant problems with adjoining floor levels.

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of an extension floor (and the whole-house perimeter rule)?

A solid floor in a 4m × 5m rear extension — 100mm of PIR over the slab under a 65mm screed — has a U-value of about 0.18 W/m²K (0.177) on the extension’s own exposed perimeter and area (13m, 20m²), inside the 0.18 Approved Document L sets for a new floor in an existing home. Approved Document L also lets the floor of an extension be calculated with the exposed perimeter and floor area of the whole enlarged dwelling (Table 4.2, note 5): for a semi-detached house that takes the same floor to about 0.17, and lets 90mm of PIR pass (0.179) where on the extension alone it misses (0.192).

Can I use the whole house’s perimeter and area for the extension floor?

Yes. Approved Document L says the U-value of the floor of an extension may be calculated using the exposed perimeter and floor area of the whole enlarged dwelling (2021 edition, Table 4.2 note 5; the 2026 edition keeps it as Table 3.2 note 5). A bigger floor with relatively less edge gives a lower U-value, so it usually helps — here 90mm of PIR meets 0.18 on the whole house (0.179) but not on the extension alone (0.192). Enter the whole house’s figures in U-Monkey’s perimeter and area boxes to check it.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Floors

Walls

Roofs

Different build-up? See the full floor U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.

Your build-up won't be exactly this one.

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