What U-values does an extension need?

Under Part L (2021) in England, new elements of an extension must meet limiting U-values of 0.18 W/m²K for walls, 0.15 for roofs, 0.18 for floors, and 1.4 for windows and doors (2.2 for rooflights).

Do I need building regulations approval for an extension?

Almost always, yes, even when planning permission is not needed. You can apply with full plans, which are checked before work starts, or with a building notice for simpler work.

How much glazing can an extension have?

Without extra calculations, windows, rooflights and glazed doors should not exceed 25% of the extension's floor area, plus the area of any windows or doors that the extension removes or covers.

Building Regulations for a house extension

Planning permission decides whether you can build; Building Regulations decide how. Almost every extension needs Building Regulations approval, even when it doesn't need planning permission. Here's what's involved in England.

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Full plans or building notice?

Full plans: drawings and a specification are checked and approved before work starts. Best for extensions, and needed if you want certainty before you build.
Building notice: you give notice and the inspector checks the work as it goes. Fine for simple work, riskier for an extension.

You can use your council's Building Control or a registered private Building Control approver. Either way the work is inspected at stages: foundations, damp-proof course, drainage, structure and completion.

Insulation: Part L U-values

New parts of an extension must meet these limiting U-values (W/m²K, Part L 2021). The lower the number, the better the insulation.

Element	Limit	Typical build-up
External walls	0.18	Brick and block cavity wall with 150 mm full-fill mineral wool
Pitched roof	0.15	About 300 mm of mineral wool at ceiling level, or PIR between and under rafters
Flat roof	0.15	Warm deck with about 140 mm PIR
Ground floor	0.18	Concrete slab with about 100 mm PIR (depends on the floor's shape)
Windows and doors	1.4	Double glazing, low-E glass, argon filled
Rooflights	2.2

Glazing is limited to 25% of the extension's floor area, plus the area of any windows or doors the extension removes, unless you show compliance another way.

The other parts that usually apply

Part	What it means for an extension
A — Structure	Foundations, beams, lintels and roof members designed to carry their loads. Steel beams over new openings need a structural engineer's calculations.
B — Fire	Mains-powered, interlinked smoke alarms; escape windows from first-floor habitable rooms; fire resistance near boundaries.
C — Moisture and radon	A damp-proof course and membrane, and radon protection where the UKradon map shows it is needed.
F — Ventilation	Extract fans in kitchens and bathrooms, trickle vents in windows, and opening windows for purge ventilation.
G and J	Hot water safety, and safe flues for boilers and stoves near the new walls and windows.
H — Drainage	New drains and rainwater, and building over or near existing drains.
K, M and Q	Safety glass, access that is no worse than before, and secure doors and windows.
P — Electrics	Electrical work by a registered competent person, with a certificate.

Other things to arrange

Structural calculations from an engineer for beams, lintels and roof members.
Party Wall notices if you build on or near a shared boundary or wall.
Build-over agreement with the water company if the extension is near a public sewer.

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Extension Plans draws 1:50 sections through each extension, annotated floor plans with alarms, fans, steels and escape windows, and a full specification, with the insulation sized to pass Part L. Upload your floor plan and the house is built for you.

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A guide only. Building Control will need the drawings checked and completed by a professional, and a structural engineer's calculations. The specification follows the Approved Documents for England; check they are current.