Exempted development
Can I build a garden room without planning permission?
A garden room goes by plenty of other names: a garden studio, a home office, a cabin, a summerhouse, a shed, a garage, a store or a greenhouse. Planning law uses none of them. It treats them all as one thing, a structure within the curtilage of a house, and applies the same rules to each: Class 3 exempted development. If yours stays inside the 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
Exempted development is not a single rule but a long schedule of classes, running from gates and walls to agricultural buildings. Three of them account for most domestic work: Class 1, extending a house; Class 3, a garden room, shed or garage; and Class 3A, a detached house in the rear garden. This guide covers those three and no others. The checker above applies Class 3.
These come from Class 3 of Schedule 2, Part 1 of the Planning and Development Regulations 2001. They cover a garden room, shed, garage, greenhouse, store or similar structure within the grounds of a house. They do not apply to an apartment.
- Behind the front wall
- Nothing under Class 3 may stand forward of the front wall of the house. To the rear or the side is fine; a front garden needs permission.
- 30 square metres in total
- That is the total for every shed, garage, greenhouse and garden room in the grounds together, not per structure. If there is already a 12 m² shed, you have 18 m² left. The limit was 25 m² until 27 July 2026.
- 25 square metres of private open space
- Open space must remain to the rear or side of the house for the people living in it. Unlike a rear extension, where the whole site now counts, a front garden does not count here. A garden room and a rear extension draw on the same space, so one can use up the room for the other.
- 3 metres high, or 4 with a tiled or slated pitched roof
- Measured to the highest point. A pitched roof in any other material, such as felt or metal, gets the 3 m figure.
- Matching finishes at the side
- A garage or structure to the side of the house must match the house in its external finishes, and in its roof covering if the roof is tiled or slated. The checker cannot see this; it is worth knowing.
- Not for living in
- The structure cannot be used for human habitation, or for keeping pigs, poultry, pigeons, ponies or horses. An office, studio, gym or store is fine. A bedroom or a self-contained unit is not.
Class 3A: a detached house in the rear garden
Class 3 rules out living accommodation. A separate class, inserted by S.I. No. 340 of 2026, allows the thing Class 3 does not: a detached house in the rear garden of a house, occupied alongside the main house rather than instead of it. It is a separate exemption on its own conditions, and the checker above does not apply it.
On size, it works from both ends. The new house must be at least 32 square metres, and taken together with any Class 3 structures already in the curtilage it cannot exceed 45. A garden room built today therefore eats into what is left for a Class 3A house later. The height limits are the same as Class 3, 3 metres or 4 with a tiled or slated pitched roof. At least 25 square metres of private open space must remain, measured across the whole curtilage rather than the rear and side only. It has to stand at least 0.6 metres off any wall or party boundary, with any window the same distance from the boundary it faces, and it cannot take a new access onto the road or its own connection to water or wastewater.
The conditions on its use are as firm as the ones on its size. It can only be occupied in conjunction with the main house, cannot be sold or subdivided away from it, and cannot be let short term. The principal house must be the owner's sole or main residence when the work starts, and must not itself have been subdivided. Because the result is a house, the Building Regulations apply in full, fire safety included. The planning authority has to be notified, with the location and Eircode, at least 14 days before work begins. The exemption is also temporary: it covers work both started and finished by 31 December 2030.
Last updated 29 September 2026. The figures on this page are current as of two instruments, both in operation from 27 July 2026. S.I. No. 338 of 2026 substituted Class 3, raising the total area limit from 25 m² to 30 m² and leaving the other conditions as they were. S.I. No. 340 of 2026 inserted Class 3A, which did not exist before.
What this tool does not check
The checker tests the limits that can be worked out from dimensions: total area, open space and height. It cannot see the finishes of a structure at the side of the house, 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.
Planning a rear extension as well? That is Class 1, the third of the classes this guide covers. The rear extension checker applies those 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
Can I build a garden room without planning permission in Ireland?
Usually, yes. A garden room, shed, garage or similar structure within the curtilage of a house is exempted development under Class 3 if it is behind the front wall of the house, all such structures together do not exceed 30 square metres, at least 25 square metres of private open space is left to the rear or side, it is no more than 3 metres high (4 metres with a tiled or slated pitched roof), and it is not used as living accommodation.
Is the limit 25 or 30 square metres?
It is 30 square metres. The limit was 25 square metres for many years and was raised to 30 by S.I. No. 338 of 2026, which came into operation on 27 July 2026. The figure is a total for every shed, garage, greenhouse and garden room in the curtilage together, not per structure.
Can I use a garden room as a home office?
Yes. A home office, studio, gym, workshop or store is a use incidental to the enjoyment of the house, which is what Class 3 allows. What it does not allow is human habitation. Sleeping accommodation, a granny flat or a unit to rent out falls outside Class 3 whatever its size. A detached house in the rear garden is a separate question, dealt with by Class 3A since 27 July 2026, on conditions of its own.
Can I build a granny flat in my back garden without planning permission?
Possibly, under Class 3A, inserted by S.I. No. 340 of 2026 and in operation from 27 July 2026. It allows a detached house in the rear garden of a house: at least 32 square metres, and no more than 45 square metres once any shed or garden room already in the curtilage is counted with it. It can only be occupied in conjunction with the main house and cannot be sold, subdivided or let short term. The Building Regulations apply in full, the planning authority must be notified at least 14 days before work starts, and the exemption only covers work both started and finished by 31 December 2030. The checker on this page applies Class 3 and does not test Class 3A. Ask us if you are considering one.
How high can a garden room be without planning permission?
3 metres to the highest point, or 4 metres if it has a tiled or slated pitched roof. A pitched roof in any other material, such as felt or metal, gets the 3 metre figure.
Does a garden room affect how big an extension I can build later?
It can. A rear extension under Class 1 and a garden room under Class 3 both have to leave at least 25 square metres of private open space, and the same garden serves both. A garden room that uses up the spare space can rule out a later extension, or the other way round, so it is worth planning the two together.
Can I put a garden room in my front garden?
Not under Class 3. Condition 1 says no such structure may be placed forward of the front wall of the house. A structure in a front garden needs planning permission.
Does a garden room need to comply with building regulations?
Planning permission and the Building Regulations are separate systems. A small detached structure that is not lived in is often exempt from most of the Building Regulations too, but a garden room with electricity, insulation and drainage should be built to them, and anything used regularly as a workspace is worth getting right. Ask us if you are unsure.
The checker says my garden room is exempt. Is that a decision?
No. It is a guide based on the limits that can be checked from dimensions. A structure at the side of the house must also match the house in its external finishes and roof covering, and exempted development does not apply at all to protected structures, in some Architectural Conservation Areas, or where a planning condition removed it. For certainty, a local authority can issue a declaration under Section 5 of the Planning and Development Act 2000, and we can prepare that application for you.
Thinking about a garden room?
Terry & O'Flanagan Ltd has been getting people planning permission in Dublin and the surrounding counties since 2000. 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 S.I. 340/2026, and reviewed by a registered architect, but it remains a guide rather than professional advice.