When a house extension does not need planning permission in Ireland

Published 21 September 2026 · Last updated 21 September 2026 · The Home Upgrade Co.

The rule lives in Class 1 of Part 1 of Schedule 2 of the Planning and Development Regulations 2001, and it was rewritten by the Planning and Development (Exempted Development (Act of 2000)) Regulations 2026, S.I. No. 338 of 2026. The same day brought a bigger garden room limit and a brand new exemption for a small detached house in the back garden, so people usually end up asking about all three at once. All three are below, with the conditions in full.

Plain about who we are: The Home Upgrade Co does not build extensions and we are not planning consultants. We fit the kitchens, windows, doors and wardrobes that go inside them, which is why we keep this page accurate. Your architect, engineer or builder confirms the rules against your own site before anything starts, and the council can give you a formal answer under section 5 of the Planning and Development Act 2000 if there is any doubt.

What size extension can you build without planning permission in Ireland?

Up to 45 square metres, to the rear of the house, as long as every other Class 1 condition is met. That total includes any extension already built since 1 October 1964.

The wording substituted by S.I. 338 of 2026 reads that where the house has not been extended previously, the floor area of any such extension shall not exceed 45 square metres. Citizens Information puts the same thing in plain English and confirms the history: the exemption applies where the work does not increase the original floor area of the house by more than 45 square metres, a limit increased from 40 square metres on 27 July 2026.

Floor area is the whole extension, not just the new living space, and it is measured cumulatively across the life of the house. Go one square metre over and the whole thing needs permission, not just the excess.

When exactly did the limit change from 40 to 45 square metres?

On 27 July 2026. The regulations were signed on 16 July 2026 and came into operation on 27 July, alongside six other sets of exemption changes.

The Department of Housing, Local Government and Heritage announced it on the day, confirming that the existing exemption for extension of the principal dwelling will be increased from 40 square metres to 45 square metres. Seven statutory instruments, S.I. 338 to 344 of 2026, were made together and all commenced on the same date.

This matters for more than the extra five square metres. If you were quoted or advised before that date, or you are reading a guide that has not been updated, the figure in front of you is probably the old one.

Does the extension have to be at the back of the house?

Yes. Class 1 covers an extension to the rear of the house only, plus converting an attached garage, store or shed at the rear or the side into part of the house.

There is no exemption for extending out the front. The only thing you can add at the front without permission is a porch, and that is capped at two square metres and must sit more than two metres back from any public road or footpath. A side extension is not exempt either, although converting an existing attached garage on the side counts, because that structure is already there.

Is the limit different for a terraced, semi detached or detached house?

Only above ground level. The 45 square metre total is the same for every house, but the first floor part of it is capped at 12 square metres for a terraced or semi detached house and 20 square metres for a detached house.

Those sub limits sit inside the 45, they are not added to it. So a semi detached house could take 33 square metres at ground level and 12 above, and that is the lot. Any extension above ground level also has to be not less than 2 metres from any party boundary, which in practice rules out a first floor extension tight against the neighbour on a standard estate plot.

Do previous extensions count towards the limit?

Yes, and this is where most people come unstuck. Every extension built since 1 October 1964 counts, including ones that were granted planning permission.

The regulation is explicit that the floor area is taken together with any previous extension constructed after 1 October 1964, including those for which planning permission has been obtained. A house that already has a 30 square metre kitchen extension from 2008 has 15 square metres of exemption left, not 45. A previously granted permission does not reset the clock, and neither does a change of owner.

Before you measure anything, find out what has already been added. The council planning register and the deeds are the usual places to look, and a previous extension that nobody mentioned is the single most common reason an assumed exemption turns out not to exist.

How much garden do you have to leave?

At least 25 square metres of private open space for the people living in the house. This is a floor, not a guideline, and a big extension on a small plot fails on it long before it fails on floor area.

The 2026 wording tightened slightly. The old condition measured the open space left to the rear of the house. The new one measures the total area of private open space within the curtilage of the house, which means the whole site rather than just the back garden. Note too that a garden room or a garden house eats into the same 25 square metres, so the three exemptions on this page compete with each other for the same ground.

How close to the boundary can the windows be?

Ground floor windows in the extension must be at least 1 metre from the boundary they face. Windows above ground level must be at least 8 metres, reduced from 11 metres in 2026.

That 11 to 8 metre change is a quiet but useful one, because the old figure made a first floor rear window on a normal suburban garden almost impossible to keep exempt. It is still a real constraint. Measure from the window to the boundary it looks at, not to the nearest boundary.

How high can an exempt extension be?

No higher than the house. The walls cannot exceed the height of the rear wall, or the side walls where the rear wall includes a gable, and the roof cannot go above the highest part of the house roof.

For a flat roofed extension the ceiling is the height of the eaves or parapet, whichever applies. One more condition that catches people out: the roof of the extension cannot be used as a balcony or a terrace. If the plan involves standing on it, it is not exempt.

What is exempt and what still needs permission?

The table below is the short version of the three exemptions people ask about together. Every row still depends on the full conditions, and on the restrictions further down this page.

What size garden room can you build without planning permission?

Up to 30 square metres, raised from 25 square metres on 27 July 2026. That is the running total for every shed, garage, greenhouse and garden room in the curtilage, not each one.

This is Class 3, also rewritten by S.I. 338 of 2026. The structure cannot go forward of the front wall of the house, it cannot exceed 4 metres in height with a tiled or slated pitched roof or 3 metres with any other roof, and it must leave that same 25 square metres of private open space. Where it sits to the side of the house, the finishes and roof covering have to match the house. The Department confirmed the increase, stating that the existing exemption for a structure in the back garden will be increased from 25 square metres to 30 square metres.

Can you live in a garden room?

No. Class 3 says the structure cannot be used for human habitation, and that condition survived the 2026 rewrite untouched.

The exact wording is that the structure shall not be used for human habitation or for any purpose other than one incidental to the enjoyment of the house. A home office, a gym, a playroom or storage is fine. A bedroom, a self contained unit or anything let out is not. If a supplier is selling you a 30 square metre garden room as a place for someone to sleep, they are selling you something that needs either planning permission or the Class 3A route below.

Can you build a granny flat in the back garden without planning permission?

Since 27 July 2026, yes, under a new Class 3A, if it is between 32 and 45 square metres and you meet a long list of conditions including living in the main house yourself.

Class 3A was inserted by the Planning and Development (Exempted Development (Act of 2000)) (No. 3) Regulations 2026, S.I. No. 340 of 2026, and it covers the construction of a detached house in the rear garden of a principal house. These are the conditions, taken from the regulation itself:

• Rear garden only, and at least 32 square metres. Taken together with anything already built under Class 3 in the same rear garden, the total cannot exceed 45 square metres.
• Height up to 4 metres with a tiled or slated pitched roof, 3 metres with any other roof.
• At least 0.6 metres from any wall or party boundary, and any windows at least 0.6 metres from the boundary they face.
• It must not reduce private open space for the main house below 25 square metres.
• The principal house must be the sole or main residence of the property owner when work starts.
• It may only be occupied in conjunction with the main house, and cannot be sold or subdivided away from it.
• It cannot be used for short term letting.
• No separate connection to utilities, including water or wastewater. It runs off the main house.
• It cannot be temporary, so a caravan or a mobile home does not qualify.
• No new vehicular or pedestrian access onto a road, and independent pedestrian or wheelchair access must be provided within the curtilage.
• It cannot be used where the principal house has already been subdivided.
• A notification must go to the planning authority not less than 14 days before work starts, giving the location and the Eircode.
• Work must be commenced and completed inside the relevant period, which ends on 31 December 2030.

One thing that is genuinely not settled yet. The Department said on the day that the tax arrangement for such auxiliary dwellings and how it interacts with the Rent-a-Room tax relief will be clarified during this year's Budgetary process. Several suppliers are already telling people these units can be rented under Rent-a-Room. As of the date on this page that has not been confirmed, so treat it as an open question and take tax advice.

Do you have to tell the council before you start?

Not for an ordinary extension. The 14 day notice applies only to a detached garden house under Class 3A and to subdividing a house under Class 1A.

There is no notification condition anywhere in Class 1 or Class 3, so an exempt rear extension or a garden room needs nothing sent to the council before work begins. Citizens Information sets out the notice requirement for the two new classes only, confirming that you must notify your local authority about adding a home to your garden at least 14 days before the work starts.

If you want certainty rather than an opinion, section 5 of the Planning and Development Act 2000 lets you ask the council for a formal declaration on whether a specific proposal is exempted development. That is worth doing whenever the arithmetic is close or the history of the house is unclear.

Does a protected structure or a conservation area change it?

Yes, and it overrides everything above. Works to a protected structure are exempted development only where they would not materially affect the character of the structure.

That comes from section 57 of the Planning and Development Act 2000, which applies notwithstanding the exempted development regulations. An owner or occupier can ask the planning authority for a written declaration on what would and would not materially affect the character, and the authority has 12 weeks to issue it.

Architectural conservation areas are covered separately. Under article 9 of the Planning and Development Regulations 2001, exemptions do not apply where external works in an architectural conservation area would materially affect the character of the area. The same article removes the exemption where the work would contravene a condition of an existing planning permission, or where it extends an unauthorised structure. A condition on the original estate permission removing permitted development rights is common enough to be worth checking for.

Do the Building Regulations still apply?

Yes, in full. Planning exemption and building control are two separate systems, and being exempt from one has never meant being exempt from the other.

Structure, fire safety, ventilation, drainage and energy performance all still apply. Class 3A spells it out for the avoidance of doubt, stating that the relevant building control legislation shall apply, such as Building Regulations including Fire Safety, and Building Control Regulations.

There is a second threshold here that moved on the same day and almost nobody has reported. The Building Control (Amendment) Regulations 2026, S.I. No. 359 of 2026 raised the floor area that triggers full statutory certification from 40 square metres to 45, expressly to align with the new planning figure. The Building Control Regulations now apply the commencement notice, design certifier and assigned certifier machinery to a new dwelling or an extension to a dwelling involving a total floor area greater than 45 square metres. Had that not been changed, a 45 square metre extension would have been exempt from planning and caught by certification at the same time.

Two details worth knowing. For that building control threshold, total floor area means only the extension you are proposing, not the cumulative figure the planning rule uses. And because a Class 3A garden house is a new dwelling rather than an extension, it falls inside the certification regime regardless of its size.

Who fits out the inside once the shell is up?

That is where we come in. Your builder does the structure, and we design, supply and fit what goes inside it.

An exempt extension is usually a kitchen, a family room or both, and the fit out decisions want to be made at drawing stage rather than after the builders leave. We design the kitchen in 3D from your architect plans, so the plumbing, waste and electrics go into the right places first time, and we supply and fit the windows, patio doors, internal doors, fitted wardrobes and fitted furniture that finish the room. If the work is an attic conversion rather than an extension, our wardrobes for attic and sloped ceilings are made to measure for the awkward angles, and a garden room used as a study takes a made to measure home office better than anything flat packed.

There is more on sequencing and layout in our guide to kitchen extension ideas that work in Irish homes. We cover Dublin, Meath, Kildare and Wicklow.

What to do next

Work through it in this order, because each step can stop the one after it.

• Find out what has already been built. Every extension since 1 October 1964 comes off your 45 square metres, permitted or not.
• Measure the garden that will be left. If it drops below 25 square metres of private open space, the exemption is gone whatever the floor area says.
• Check whether the house is a protected structure or sits in an architectural conservation area, and read the original planning permission for conditions.
• Decide which exemption you are actually using: the extension under Class 1, a garden room under Class 3 that nobody can live in, or a garden house under Class 3A with its 14 day notice.
• If it is close, ask the council for a section 5 declaration rather than relying on anybody's opinion, including ours.
• Talk to your builder about building control, and get the kitchen and joinery designed before the floor goes down.

Frequently asked questions

What size extension can I build without planning permission in Ireland?

Up to 45 square metres to the rear of the house, provided every other condition in Class 1 is met. The limit rose from 40 square metres on 27 July 2026.

Do previous extensions count towards the 45 square metre limit?

Yes. Anything built since 1 October 1964 counts, including extensions that were granted planning permission. The 45 square metres is the running total for the house, not a fresh allowance.

Do I need to tell the council before I start an exempt extension?

No. There is no notice requirement for an exempt extension under Class 1. The 14 day notice applies only to a detached garden house under Class 3A and to subdividing a house under Class 1A.

Can I live in a garden room?

No. A Class 3 garden room, shed, home office or gym cannot be used for human habitation, whatever a supplier tells you. A garden building someone sleeps in has to meet the separate Class 3A rules instead.