Exempted development
How big can I build without planning permission?
Most rear extensions in Ireland fall under something called Class 1 exempted development. If your extension stays inside its limits, you may be able to build without a planning application. Size one below and watch the limits as you go.
The rules, in plain English
These come from Class 1 of Schedule 2, Part 1 of the Planning and Development Regulations 2001. They apply to an extension to the rear of a house, not to the side or front, and not to an apartment.
- 45 square metres of floor area
- That is the total for every extension built since 1 October 1964, not per project. If a previous owner added 20 m², you have 25 m² left. The limit was 40 m² until 27 July 2026.
- 25 square metres of private open space
- Open space must remain within the curtilage for the people living in the house. Until July 2026 only the space behind the house counted; it is now the total across the whole site, so a front or side garden counts too. On a tight site this still runs out well before the floor area does, which is why it is the condition people most often trip over.
- 12 or 20 square metres above ground level
- For anything above the ground floor: 12 m² on a terraced or semi-detached house, 20 m² on a detached one. This sits inside the 45 m², it is not added to it. Above-ground work must also be 2 m clear of a party boundary.
- No higher than the rear wall
- The walls of the extension cannot stand above the rear wall of the house. A flat roof cannot exceed the eaves or parapet; a pitched roof cannot exceed the ridge of the existing roof.
- Windows kept away from boundaries
- A window at ground level must be at least 1 m from the boundary it faces. Above ground level, that distance is 8 m, reduced from 11 m in July 2026. The roof of the extension cannot be used as a balcony or terrace.
- The main house only
- Added in July 2026: the exemption applies to the principal house on the site. A second, detached house within its grounds cannot use it.
Last updated 10 September 2026. The figures on this page are current as of S.I. No. 338 of 2026, the Planning and Development (Exempted Development (Act of 2000)) Regulations 2026, which substituted Class 1 and came into operation on 27 July 2026. It raised the floor area limit from 40 m² to 45 m², reduced the above-ground window distance from 11 m to 8 m, changed private open space from a rear-only to a whole-curtilage measure, and added the principal-house condition.
What this tool does not check
The checker tests the limits that can be worked out from dimensions: floor area, open space, height, and the party boundary distance. It cannot see where your windows go, what shape your roof is, whether your house is a protected structure, or what conditions were attached to the permission your estate was built under. Those are all capable of removing the exemption on their own.
Thinking about a garden room, shed or garage instead? The garden room checker applies the Class 3 rules, and the two share the same 25 m² of open space.
If you need certainty rather than a guide, a local authority can issue a formal declaration under Section 5 of the Planning and Development Act 2000 confirming whether your proposal is exempt. Preparing that application is part of what we do.
Common questions
How big an extension can I build without planning permission in Ireland?
Up to 45 square metres of total floor area to the rear of a house, provided at least 25 square metres of private open space is left within the curtilage, the extension is no higher than the rear wall, and the house has not already used up the allowance. The 45 m² is cumulative across every extension built since 1 October 1964, so a previous extension counts against it. The limit was 40 square metres until S.I. 338 of 2026 raised it on 27 July 2026.
Is the extension limit 40 or 45 square metres?
It is 45 square metres. The limit was 40 square metres for many years, and was raised to 45 by S.I. No. 338 of 2026, which came into operation on 27 July 2026. A great deal of guidance still online quotes the old 40 square metre figure, so it is worth checking the date on anything you read.
When did the planning exemption rules change in Ireland?
27 July 2026. The Planning and Development (Exempted Development (Act of 2000)) Regulations 2026 (S.I. No. 338 of 2026) were signed on 16 July 2026 and came into operation on 27 July 2026. They substituted Class 1 and Class 3 of Part 1, Schedule 2 of the Planning and Development Regulations 2001.
What changed in the 2026 exempted development regulations?
Four things for rear extensions. The total floor area limit rose from 40 to 45 square metres. The minimum distance from an above-ground window to the boundary it faces fell from 11 metres to 8 metres. The 25 square metre private open space requirement is now measured across the whole curtilage rather than only the area behind the house, so a front or side garden counts toward it. And a new condition limits the exemption to the principal house, so a second detached house within its grounds cannot use it. Separately, the limit for sheds, garages and similar structures rose from 25 to 30 square metres.
Does a two-storey extension count differently?
Yes. Floor area above ground level is capped separately: 12 square metres on a terraced or semi-detached house, and 20 square metres on a detached house. That cap sits inside the 45 m² total rather than being added to it. An above-ground extension must also stand at least 2 metres clear of a party boundary.
What is 'private open space' and why does it matter?
It is the open space within the curtilage of your house reserved for the people living there. At least 25 square metres must remain after the extension is built. Until July 2026 only space behind the house counted; S.I. 338 of 2026 changed it to the total across the whole curtilage, so a front or side garden now counts toward it. It is still the rule that catches most people out, because on a small site the open space requirement bites long before the 45 m² floor area limit does.
Does exempted development mean I don't need to comply with building regulations?
No. Planning permission and the Building Regulations are two separate systems. An extension can be exempt from needing planning permission and still have to meet the full Building Regulations for structure, fire safety, insulation, ventilation and drainage. A Commencement Notice may also be required.
Are there properties where none of this applies?
Yes, and it is worth checking before you measure anything. Exempted development does not apply to protected structures, is restricted in Architectural Conservation Areas, and does not apply to apartments. Many housing estates were also granted permission on condition that exempted development rights were removed. That condition is written into the original grant and is easy to miss.
The checker says my extension is exempt. Is that a decision?
No. It is a guide based on the limits that can be checked from dimensions alone. Several Class 1 conditions depend on things a checker cannot see, such as window positions relative to boundaries and the form of the roof. If you want certainty, a local authority can issue a formal declaration under Section 5 of the Planning and Development Act 2000, and we can prepare that application for you.
Thinking about an extension?
Terry & O'Flanagan Ltd has been getting people planning permission in Dublin and the surrounding counties since 2000, from single-storey rear extensions to 147-unit residential schemes. If you want to talk it through, we are easy to reach.
Get in touchTool built by Conor O'Flanagan. The planning rules it applies are summarised from S.I. 600/2001 as amended by S.I. 338/2026 and reviewed by a registered architect, but it remains a guide rather than professional advice.