House extension rules in Ireland: what changes on 27 July 2026

The rules in force today (until 26 July 2026)

Under Class 1 of the 2001 Planning Regulations, you can extend a house to the rear without planning permission if every condition is met. The big ones:

• Maximum 40 m² floor area, and that is cumulative: every extension since 1 October 1964 counts, including ones that got planning permission.
• Above ground floor (a first-floor extension), the limit within that 40 m² is 12 m² for a terraced or semi-detached house and 20 m² for a detached house, and any above-ground extension must be at least 2 m from a party boundary.
• Height limits: extension walls no higher than the house's rear wall; a flat roof no higher than the eaves or parapet; a pitched roof no higher than the house roof.
• You must keep at least 25 m² of private open space behind the house.
• Windows: at ground level at least 1 m from the boundary they face; above ground level at least 11 m; and the roof of the extension cannot be used as a balcony or roof garden.

What changes on 27 July 2026

S.I. No. 338 of 2026, signed 16 July 2026, amends Class 1 with effect from 27 July 2026:

• The limit rises from 40 m² to 45 m² - both for a first extension and cumulatively (the since-1964 counting rule stays).
• The above-ground window distance relaxes from 11 m to 8 m.
• The open-space test is reworded: at least 25 m² of private open space must remain within the curtilage as a whole, not just to the rear.
• The first-floor sub-limits (12 m² / 20 m²) and the 2 m party-boundary rule are unchanged.
• A new condition confirms the exemption applies only to the principal house - not to any additional dwelling in its garden.

Three brand-new exemptions arriving the same day

The 27 July package goes beyond bigger extensions:

• Class 1B: a dormer to the side or rear of the roof, up to 30 m³ internal volume, kept below the ridge and set back 20 cm from the eaves (S.I. 344 of 2026).
• Class 1C: up to two rooflights on the front roof slope, each up to 100 cm by 90 cm (S.I. 344 of 2026).
• Class 1A: subdividing a house into a maximum of two self-contained units, each at least 32 m² - with a 14-day prior notification to the planning authority, available for works commenced and completed by 31 December 2030 (S.I. 339 of 2026).

When the exemption does not apply

Exemptions can be stripped case by case under article 9 of the regulations: protected structures, architectural conservation areas, and situations where works would breach a planning condition or endanger public safety are the common ones. If your house is protected or in an ACA, assume you need permission until the council tells you otherwise - a Section 5 declaration (EUR 80, answered within 4 weeks) settles it formally.

Building regulations still apply - always

Planning exemption never exempts you from the building regulations: structure, fire, ventilation, energy and the rest apply to every habitable extension.

The paperwork differs by route. A planning-exempt extension needs no commencement notice. An extension built under a planning permission needs a Commencement Notice through the Building Control Management System (BCMS) 14 to 28 days before starting, for a EUR 30 fee. And any extension over 40 m² total floor area falls under the full BCAR certification regime (design certifier, assigned certifier, completion certification) - although owners of single dwellings can opt out of statutory certification under S.I. 365 of 2015, with the regulations themselves still binding.

Useful tools

Quick answers

Sources

• S.I. No. 600 of 2001, Schedule 2 (current Class 1)
• S.I. No. 338 of 2026 (45 m² from 27 July 2026)
• S.I. No. 339 of 2026 (Class 1A subdivision)
• S.I. No. 344 of 2026 (dormers + rooflights)
• Government announcement, 21 April 2026
• Building Control Regulations (S.I. 496 of 1997)

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