The 2026 edition of Approved Document L was published on 24 March 2026, but it doesn't take effect until 24 March 2027. Until that date, the rules governing your boiler replacement are still the 2021 edition, with its 2023 amendments, and that's the edition your installer's quote should be working to.

Work that straddles the date

If your job is notified to building control before 24 March 2027, it can stay under the earlier rules even if the work itself starts afterwards, as long as it begins before 24 March 2028 (Circular 01/2026). That matters if you're getting quotes now for work that might not start until next year. You can check which edition applies to your job against the dates that affect it.

A correction, not a change of substance

Circular 04/2026, published on 8 September 2026, corrected minor errors in the 2026 edition. None of what it corrected touches the rules for a replacement boiler.

What changes once the new edition starts

The efficiency minimums stay where they are, and so does the list of four extra measures a gas combi needs to come with. What changes is how the boiler gets sized and how a combi behaves once the weather turns mild.

• From 24 March 2027, a replacement boiler should be sized from a heat loss calculation covering the whole home (paragraph 4.9).
• A gas combi should be able to turn itself down to the home's typical mid-season heating need (paragraph 4.11).

The 2026 edition also renamed Approved Document L's Volume 1, from "Conservation of fuel and power" to "Energy and greenhouse gas emissions". You can read more about what the title change covers.

Your flue position doesn't move

None of this changes where a flue can terminate. Flue position is set by Approved Document J and the manufacturer's installation instructions, and neither has moved with this edition of Approved Document L. You can check where a flue is allowed to go separately from any of this.

From 24 March 2027, a replacement boiler has to be sized from a whole-home heat loss calculation. The installer works out how much heat your house loses, room by room, and sizes the new boiler to that figure.

Combi boilers covered by that date also need to be able to turn down to a mid-season load, meaning the boiler can run at a much lower output on a mild day. A boiler that only knows how to run flat out is less efficient on the mild days that make up most of the heating season, so this is about how the boiler behaves once it's fitted.

What stays the same

The efficiency minimums themselves don't move. A gas boiler still has to meet the same efficiency floor set out in Approved Document L (Table 6.2). What changes is how the installer gets there and what they have to show their working with.

If your installer gives you a quote close to that date, ask whether the sizing is based on a heat loss calculation for your house and whether the combi they've specified can turn down for mid-season running. Those are two separate questions from the efficiency figure itself, and your quote should answer all three. The checklist of what a quote should name sets out where each of these belongs: what a boiler installation quote must name.