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How big can an extension be without planning permission in Ireland?

Up to 40 m² of rear extension, subject to conditions. Class 1 of Schedule 2 to the Planning and Development Regulations 2001 exempts rear extensions to a house up to 40 m² in total, with tighter limits for work above ground-floor level and a requirement to retain private open space.

Class 1 of Schedule 2 to the Planning and Development Regulations 2001 (as amended) allows an extension to the rear of a house, provided the total floor area of that extension — taken together with any previous extension built since 1 October 1964 — does not exceed 40 m².

The allowance is not all available at first-floor level. Class 1 sets a much smaller limit for any part of the extension above ground-floor level, and that limit differs between terraced or semi-detached houses and detached houses. Upper-floor windows must also meet a minimum distance from the rear boundary.

Conditions on height, on retaining at least 25 m² of private open space to the rear, and on not extending forward of the front wall all apply. Extensions to the front or side of a house are generally not exempted development.

Key conditions to check

Class 1 applies only where all of the following hold:

  • The extension is to the rear of the house
  • Total extension floor area since 1 October 1964 stays within 40 m²
  • Any part above ground-floor level stays within the smaller limit for your house type
  • Height does not exceed the roof of the house (and the class's specific height limits)
  • At least 25 m² of private open space remains to the rear
  • Upper-floor windows meet the minimum distance from the opposite boundary
  • The house is not a protected structure and the works are not restricted by a planning condition

When to get confirmation

Because the 40 m² is cumulative, older extensions can quietly use up the allowance. If there is any earlier extension, an attic conversion with a dormer, or a boundary-distance question, confirm the position with your local planning authority or apply for a section 5 declaration before starting work.

Sources

Last reviewed 20 August 2026. General information only — not legal or planning advice.

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