Building regulations for extensions
Every extension must meet the building regulations, even one that's exempt from planning. Since 27 July 2026, an extension of up to 45 m² that is exempt from planning needs no commencement notice. Over 45 m², or if it needed planning permission, you lodge a commencement notice 14 to 28 days before work starts, and over 45 m² it comes with a design certifier and an assigned certifier, unless you opt out.
Commencement notices, BCAR certifiers and the opt-out, and what the energy rules ask of the new room.
Free, no obligation. One local builder prices it, not five.
- 45 m²building control threshold (was 40 m²)
- 14–28 daysbefore work: the commencement notice
- €30commencement notice fee
- 0.18W/m²K: new walls and floors (TGD L)
Which paperwork your extension needs
| Your extension | Commencement notice | Certifiers (BCAR) |
|---|---|---|
| Up to 45 m², exempt from planning | Not needed | Not needed |
| Up to 45 m², with planning permission | Yes, 14 to 28 days before work | Not needed |
| Over 45 m² | Yes, with the compliance documents | Design certifier and assigned certifier, or the owner's opt-out |
The threshold moved from 40 m² to 45 m² on 27 July 2026 (S.I. No. 359 of 2026), in step with the planning exemption. The Department of Housing's circular to councils is clear that the building regulations still apply to extensions with no commencement notice.
Certifiers, or the opt-out
Over 45 m², the building control rules (BCAR) ask for a design certifier and an assigned certifier, usually an architect, engineer or building surveyor, who inspect the work and sign it off with the builder at the end. For an extension to a house, the owner can instead lodge a declaration of intention to opt out of statutory certification. The work still has to comply; you just don't get the certificates, and a buyer's solicitor will ask how compliance was shown. Most people extending above the threshold appoint certifiers for that reason.
What the energy rules ask of the new room
The extension has to be insulated to the limits in Table 1 of TGD L 2022, the guidance to Part L:
- 0.18W/m²K: walls
- 0.18W/m²K: ground floor (0.15 with underfloor heating)
- 0.20W/m²K: flat roof
- 0.16W/m²K: pitched roof
- 1.4W/m²K: windows, doors and rooflights
And a bigger rule: if the extension work affects more than 25% of the outside surface of the existing house, it counts as a major renovation, and the whole house has to be brought up to the cost-optimal energy level as far as is technically, functionally and economically feasible. Ask your designer to check this early on a big extension to a small house.
Fire, ventilation and drains
- Ventilation (Part F): an extract fan in the kitchen and background ventilation in the new room.
- Fire (Part B): a safe way out, smoke alarms, and fire-resisting construction near boundaries.
- Drainage (Part H): building over or near a sewer or drain needs care and sometimes the water company's agreement.
- Structure (Part A): the engineer's design for the steel and foundations.
Plan your extension and see the cost
Nine quick taps. You see your plan build up as you go: the price range, what moves it, the planning position and the build time. Your number is only asked for at the end, if you want a real price.
A 25 m² single-storey rear extension in Ireland, mid-range, with wide glass doors and a new kitchen: €62,000 to €90,000 all in, including VAT and fees.
How this is worked out
We start from what single-storey extensions cost to build per square metre in each part of Ireland, including VAT at 13.5%, and the rates for side, wraparound, two-storey and heavily glazed extensions. Standard finishes take the lower half of the range, mid-range the upper half, and high spec or a wall of glass the glazed rate. Then we add professional fees (design, engineer, BER) and, if you want one, a new kitchen at your level of finish. Totals are rounded to the nearest €1,000. It's an estimate to plan with, never a quote: the builder prices your house after seeing it.
Questions people ask
Do I need a commencement notice for a kitchen extension?
Not if it's up to 45 m² and exempt from planning. Yes if it's over 45 m² or needed planning permission: lodge it 14 to 28 days before work starts.
What is BCAR?
The Building Control (Amendment) Regulations: for an extension over 45 m², a design certifier and an assigned certifier inspect and certify the work, unless the owner opts out.
Can I opt out of BCAR for an extension?
Yes. The owner of a house being extended can lodge a declaration of intention to opt out of statutory certification. The work must still comply with the building regulations.
What U-value does an extension wall need?
0.18 W/m²K for walls and ground floors, 0.20 for a flat roof, 0.16 for a pitched roof and 1.4 for windows, doors and rooflights, under TGD L 2022.
Did the 2026 changes affect building control?
Yes. From 27 July 2026 the threshold for the extra building control paperwork rose from 40 m² to 45 m², to match the planning exemption.
Get a free quote for your extension
Plan it in under a minute, see the cost, and one vetted builder who covers your county rings you with a proper price.