StructuralEngineer.ie's summary of the change: from 27 July 2026, the 45m2 extension exemption lets you build a rear extension of up to 45 m² without planning permission, up from 40 m². The limit includes every extension built since 1 October 1964, at least 25 m² of private open space must remain, and the height and boundary conditions still apply.
The change came in through the 2026 amendments to the Planning and Development (Exempted Development) Regulations (reported as S.I. 338–344 of 2026). This guide is based on published summaries. Check the statutory instrument on the Irish Statute Book, or ask your council, before relying on it for a specific house.
What the 45 m² exemption covers
- Rear extensions only. Side and front extensions still need permission.
- 45 m² in total, counting any extension built since 1 October 1964.
- Open space. At least 25 m² of private open space must remain behind the house.
- Height and boundaries. The existing conditions on height, and on walls near boundaries, still apply, and two-storey extensions have tighter conditions than single-storey ones.
Other 2026 changes worth knowing
- Garages, sheds and garden structures. The limit rises from 25 m² to 30 m².
- Dormers. Side and rear dormers are newly exempt within limits, reported as about 30 m³ of volume, below the ridge and set back from the eaves. Front dormers still need permission.
- Garden units. A new class covers a detached habitable unit of 32–45 m² in the rear garden, subject to conditions.
When the exemption doesn’t apply
- Estate conditions. Some estates have planning conditions that remove exempted development rights. Check the original grant for your estate.
- Protected structures and ACAs. Works can need permission where they’d otherwise be exempt.
- Over the limit. If earlier extensions plus the new one exceed 45 m², the whole new extension needs permission.
What still applies: Building Regulations and BCAR
The exemption is about planning only. The Building Regulations apply in full: structure, fire, energy, moisture and access. And the BCAR threshold stays at 40 m²: extensions over 40 m² need a Commencement Notice with compliance documentation, a designer and an assigned certifier, unless the owner opts out. Between 40 and 45 m², the two thresholds overlap, so confirm with your building control authority how they treat your extension before you start.
Bottom line
Rear extensions up to 45 m² in total no longer need planning, but the structure still needs designing and the certificate still matters at sale. To design and sign off yours, call 01 270 9764.
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Frequently asked questions
Does the 45 m² include my existing kitchen extension?
Yes. The limit is cumulative and counts any extension built since 1 October 1964. A house with a 15 m² extension from the 1990s has 30 m² of exempt allowance left.
Can I build a side extension without planning now?
No. The exemption covers rear extensions. Side and front extensions still need planning permission.
Do I still need an engineer for an exempt extension?
Yes, in practice. The exemption removes the planning application, not the Building Regulations. The foundations, the opening into the house and the roof still have to comply with Part A, and you'll want an opinion of compliance when you sell.
Sources
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