What U-Value Do I Need? Part L Targets for England Explained

Quick answer

In England, the U-value you need depends on the job. A new dwelling has to meet Table 4.1 of Approved Document L (roof 0.16, wall 0.26, floor 0.18). A new extension or other new element is held to Table 4.2 (roof 0.15, wall 0.18, floor 0.18). Existing elements that you renovate or retain are judged against Table 4.3, which asks for 0.16 in a roof, 0.30 for internal or external wall insulation, 0.55 for cavity fill and 0.25 for a floor. Use our PIR U-value calculator to check what thickness gets you there.

These values come from the 2021 edition of Approved Document L Volume 1 (with 2023 amendments). The 2026 edition takes effect on 24 March 2027 for most work (24 September 2027 for higher-risk buildings; see Circular Letter 01/2026), and its tables carry the same figures. Ask your Building Control body which edition applies to your project. This guide is about England only.

Need a quick quantity or thickness check? Use our free PIR U-value calculator. It uses the products we sell, prices the list live, and you can add it to your basket. Results are estimates — call 03330 908479 if you want us to check your sizes.

Table 4.1: a new dwelling

Element	Maximum U-value (W/m²K)
Roof	0.16
Wall	0.26
Floor	0.18
Party wall	0.20

These are limiting standards. The document notes that to meet the overall energy targets, some elements will need to be significantly better than these figures.

Table 4.2: new elements in an existing dwelling

This is the table that applies to most extensions and new roofs.

Element	Maximum U-value (W/m²K)
Roof	0.15
Wall	0.18
Floor	0.18

Some notes in the document soften these figures. The wall target can be relaxed if meeting it would cut the floor area of the room by more than 5%. The floor target can be relaxed where adjoining floor levels create a problem. For an extension floor, the P/A ratio can use the exposed perimeter and area of the whole enlarged dwelling, which often helps (see our thickness guide). Windows in extensions have their own limits, which we do not cover here.

Table 4.3: existing elements you renovate or retain

Table 4.3 has two columns. If an existing element is worse than the threshold (a), and you renovate it, it must be brought up to the improved value (b).

Existing element	(a) Threshold	(b) Improved
Roof	0.35	0.16
Cavity wall insulation	0.70	0.55
Internal or external wall insulation	0.70	0.30
Floor	0.70	0.25

If reaching the Table 4.3 improved value is not technically or functionally feasible, or would not pay back in 15 years or less, the element should be upgraded to the lowest value that is feasible and achieves a simple payback within 15 years. Generally an upgraded element should not be worse than 0.7. If a lower roof U-value would limit headroom, Note 3 says a lesser standard may apply, but the insulation plus any air gap should be at least the depth of the rafters and you should choose the lowest practicable U-value.

What counts as renovation?

Approved Document L (paragraph 11.2) treats renovation as adding a new layer such as cladding, render or dry-lining, stripping an element back to its structure and rebuilding, replacing a flat roof's waterproof membrane, or installing cavity wall insulation. Under paragraph 11.3, the whole element must be improved if more than 50% of its surface is renovated, or if more than 25% of the building envelope is renovated (a "major renovation"). Fitting insulated plasterboard to one room may not trigger it, but doing the whole wall will. Our guide to fitting insulated plasterboard to a solid wall covers the practical side.

Loft and garage conversions

Turning a loft or garage into a habitable room is a change to the energy status of the building (paragraphs 11.5 to 11.7). The existing thermal elements that you keep should then meet Table 4.3. For a loft room, that points to 0.16 in the sloping roof, subject to the headroom note above. Our guide to the U-value of 100mm PIR has typical build-ups.

What does that mean in PIR thickness?

PIR at λ 0.022 W/mK gives R 2.25 per 50mm, 3.40 per 75mm, 4.50 per 100mm, 5.45 per 120mm and 6.80 per 150mm. A 0.18 wall needs a total R of about 5.6, a 0.16 roof about 6.25 and a 0.30 wall about 3.3, but the other layers in your construction count towards that. The calculator accounts for them; the manufacturer's data and a designer's calculation confirm it.

Scotland, Wales and Northern Ireland

This guide is about England. Wales uses its own version of Approved Document L, Scotland uses Section 6 of the Technical Handbooks, and Northern Ireland uses Technical Booklets F1 and F2. Their U-value targets differ from those above, so check the document that applies to your site.

Frequently asked questions

Does a 0.18 floor mean the same board everywhere?

No. For ground floors the answer depends on the floor's perimeter-to-area ratio as well as the board, so a small extension needs more insulation than a large house for the same target. See our worked examples.

Do I need to upgrade the whole wall if I only insulate one room?

Not necessarily. The 50% element and 25% envelope tests in paragraph 11.3 decide that, but check with Building Control before starting.

Where do I find the official wording?

The GOV.UK Approved Document L page has both the 2021 edition in force now and the 2026 edition.

Order PIR insulation

Browse our PIR insulation boards, including floor boards and roof insulation, then check the thickness in the PIR U-value calculator. We have a £400 ex VAT minimum, with free delivery in England. British Gypsum (including Gypframe), Celotex and Knauf can also go to mainland Wales and mainland Scotland (£400 ex VAT minimum plus £40 + VAT delivery). Access panels can also go to mainland Wales and mainland Scotland, with their own charges (£15 + VAT under £300 ex VAT, free from £300). Other ranges, including suspended ceiling tiles and grid, are delivered in England only. We do not deliver to Northern Ireland or the Scottish islands.

Sources: Approved Document L (GOV.UK); Approved Document L Volume 1, 2026 edition (PDF). Figures are indicative and correct at the time of writing; they are not a substitute for a calculation by your designer or the manufacturer's technical team, and Building Control has the final say.