What size extension can I build without planning permission in Ireland?

Class 1 of Schedule 2 to the Planning and Development Regulations 2001, as amended, is the exemption most homeowners rely on for extending a house. It allows an extension to the rear of a house — meaning it does not project forward of the front wall or any wall facing a road — to be treated as exempted development where the total floor area of the extension, together with any previous extensions to the original house, does not exceed 40 m².

The 40 m² allowance is reduced in some circumstances: if the extension is two-storey rather than single-storey, or if the house is a terraced or semi-detached property in certain configurations, the cap and other conditions can be tighter, and additional restrictions apply near shared boundaries. Height limits also apply — a single-storey extension is generally capped around 4 metres, with lower limits close to a boundary, and any extension above a certain height or with specific roof features can lose the exemption.

The exemption also requires that the extension leaves adequate private open space — generally at least 25 m² — remaining to the rear of the house after the works are complete, and it does not apply where the property is a protected structure, is in an architectural conservation area, or where a condition on an earlier planning permission restricts further extension. Where any of these conditions are not met, full planning permission is required regardless of the floor area involved.