Single-storey rear extensions in Ireland
Reviewed October 2026 · Not legal, planning or financial advice
A single-storey rear extension — usually a bigger kitchen, dining and family space opening to the garden — is the most common extension in Ireland, and the one builders price most confidently.
Typical size
Most are between 15m² and 40m²: the full width of a semi-detached house (around 6–8m) and 3–5m deep. Staying at or under 40m² — counting any earlier extension — keeps the exemption possible and avoids full BCAR.
Design points that matter
- The room behind the extension loses its window to the garden. Rooflights or a lantern bring light back into the middle of the plan.
- Opening the existing rear wall needs a steel beam designed by an engineer.
- Drains often run across the back of the house; moving them adds cost.
- A flat roof with a parapet keeps the height down; a pitched roof can sit better with older houses but must respect height limits.
Long gardens
A long garden makes the 25m² open-space rule easy to meet, so the 40m² limit becomes the constraint. A deeper extension than 40m² will need permission — often worth it if the space is really needed.
Short gardens
On short plots the 25m² rule bites first. A side return or attic conversion may give space without the garden.
Regulations and their interpretation change. This guide explains the position as we understand it in October 2026. Confirm what applies to your house with an architect, planning consultant or your local authority before relying on it.