Planning permission for an extension in Ireland — the 45 m² exemption and a 94% grant rate

The seven Greater Dublin Area councils we index granted 2,061 of the 2,189 extension applications they decided in the 12 months to 12 September 2026, a rate of 94%. Dún Laoghaire-Rathdown decided 320 and refused 14 (96%); Fingal, with more rural sites, refused 23 of 152 (85%). Most extensions never reach the register at all. Since 27 July 2026 a rear extension of up to 45 m² is exempted development under Class 1 of Schedule 2, Part 1 of the Planning and Development Regulations 2001, as amended by SI 338–344/2026. The exemption is a list of conditions, not a single number. The conditions are where people go wrong.

What changed on 27 July 2026

The Planning and Development (Exempted Development) Regulations 2026, SI 338 to 344/2026, came into force on 27 July 2026. South Dublin County Council's summary of the package lists the changes that touch a house. The exempt floor area for a rear extension rose from 40 m² to 45 m². Garden structures rose from 25 m² to 30 m². Two new classes arrived: Class 1A, which lets you subdivide a house into one extra self-contained unit of at least 32 m², and Class 3A, a detached dwelling of 32–45 m² in the rear garden. Both carry a 14-day notification to the council before work starts and a completion date of 31 December 2030. The same package added an exemption for "dormer roof boxes and roof lights", covered in the attic and dormer guide.

Garden dwellings have their own guide. What follows is the ordinary extension under Class 1.

What the extension exemption requires

Class 1 exempts "the extension of a house, by the construction or erection of an extension (including a conservatory) to the rear of the house". Everything turns on the conditions attached to it.

• Rear only. A side or front extension needs an application, however small.
• 45 m² in total, ever. The new extension, together with any extension built since 1 October 1964, must stay within the limit. A house with a 30 m² kitchen extension from the 1990s has 15 m² left, not 45.
• Upstairs is tighter. Floor area above ground level counts against 12 m² for a terraced or semi-detached house and 20 m² for a detached one.
• 25 m² of private open space must remain behind the house afterwards.
• Height and windows. The extension may not rise above the house. A ground-floor window facing a boundary must be at least 1 m from it; a first-floor window at least 11 m.
• Materials and roofs must be consistent with the house.

Read the class itself before relying on it. The wording matters and the 2026 SI amended it.

When the exemption does not apply at all

Article 9 of the 2001 Regulations lists the cases where an exemption is lost even though every Class 1 condition is met. Four of them catch householders. Your house is a protected structure and the works would affect its character; you can ask the council for a free s.57 declaration on what is covered. Your house is in an Architectural Conservation Area and the works would affect the character of the area. The works would breach a condition of an earlier permission, which is common on estates built since the 1990s where the parent permission removed exempted-development rights. Or the works would obstruct a public right of way, endanger road safety or need environmental assessment.

If any of those might apply, ask the council for a s.5 declaration (€80, four weeks) before you build. A purchaser's solicitor will ask for the same document when you sell.

If you do need to apply

An extension application is an ordinary permission under s.34 of the Planning and Development Act 2000. The fee for works to an existing house is €34 (Schedule 9). You publish a newspaper notice and erect a site notice within the two weeks before lodging. Neighbours have five weeks from the day the council receives the application to make an observation for €20. The council decides within eight weeks unless it asks for further information (FI). Anyone who made an observation can appeal to An Coimisiún Pleanála (formerly An Bord Pleanála) within four weeks of the decision. Each of those steps has its own guide.

Two cautions. The counts include only decisions we could classify as a grant or a refusal; invalid, withdrawn and open applications are left out. And "extension" is matched on the applicant's description, so "two-storey extension and attic conversion" counts while "single-storey rear return" does not.

What the refusals have in common

We read the extension refusals on the Dún Laoghaire-Rathdown and Fingal registers from the summer of 2026. Almost none is a plain single-storey rear extension. They are side elements, first-floor elements and roof works. Dún Laoghaire-Rathdown refused D26B/0320/WEB at 12 Atkinson Drive, Belmont, on 16 July 2026: three flat-roofed dormers to the side roof slopes. The rear ground floor is rarely the problem. Overlooking from above, loss of light to the house next door and a design out of keeping with the terrace are.

Our reading: if the design needs a side element or anything at first floor, apply rather than argue exemption later. A retention application for the same works is refused about four times as often as one lodged in advance (see the retention guide), and the refusal sits on the register when you sell.

Two things people forget

Building Regulations apply whether planning does or not. An exempt extension still has to comply on structure, fire, ventilation and energy. A Commencement Notice on the Building Control Management System may be needed before you start, and an extension over 40 m² falls inside the 2014 certification regime with an assigned certifier. Ask the building control section.

Keep the evidence. When you sell, your solicitor will be asked for an architect's or engineer's opinion that the works were exempt or built to the permission. Drawings, dated photographs, the s.5 declaration if you got one, and the Commencement Notice acknowledgement make that opinion easy to give.

Check what your neighbours built

Search the Explorer around your own address to see what was granted and refused on your street: Blackrock, Sandyford or type any address into Find. The Dún Laoghaire-Rathdown place page carries the council's live grant rate and open observation windows; the Dundrum page does the same for that electoral area. Open windows near you are on the deadlines page.

Figures from the register on 12 September 2026 (live callouts refresh hourly); law checked 12 September 2026. Check with the council or a planning consultant before relying on this.

Frequently asked questions

Do I need planning permission for a 40 m² extension in Ireland?

Not usually. It must be to the rear, all extensions since 1964 must stay within 45 m² (from 27 July 2026), 25 m² of garden must remain, and the height, window and boundary conditions of Class 1 must be met. Protected structures, some conservation areas and estates whose permission removed exemptions are the exceptions.

Does the 45 m² limit include my existing extension?

Yes. Every extension built since 1 October 1964 counts against the limit.

How much does planning permission for an extension cost?

The council's fee is €34 for works to an existing house (Schedule 9 of the 2001 Regulations). Newspaper and site notices, drawings and any consultant's fees are on top.

How long does it take?

Eight weeks from receipt of a valid application. A further-information request adds the time you take to reply plus four weeks. If nobody appeals, the final grant issues after the four-week appeal period.

Can my neighbour stop an exempt extension?

Not through the planning system, because there is no application to observe on. A neighbour who believes the works are not exempt can ask the council for enforcement, or seek a s.5 declaration on whether the works are exempt.