Do I need planning permission for a 40 square metre extension?

No -- a rear, ground-floor extension of up to 45 square metres can be exempt under Class 1 of Schedule 2 since 27 July 2026, when SI 338/2026 raised the limit from 40 square metres. The 45 square metres includes every previous extension since 1964, and conditions on height, garden space, and windows apply. Exceed any condition and the whole extension needs permission.

Can I build a first-floor extension without planning permission?

Only within much tighter limits. Extensions above ground-floor level are capped at 12 square metres for terraced and semi-detached houses and 20 square metres for detached houses, within the overall 45 square metre allowance, and must be at least 2 metres from any boundary.

Does an attic conversion need planning permission?

A conversion using only internal works and roof windows flush with the roof is generally exempt. Dormer windows, raising the roof, or a new external stairs normally need planning permission.

Planning Permission for Extensions in Ireland

When you need planning permission for a house extension in Ireland: the 45 sqm exemption rule (raised from 40 sqm in July 2026), above-ground limits, application process, fees, and party wall considerations.

Planning Permission for House Extensions in Ireland

Extending your home is one of the most common building projects in Ireland. Whether you need planning permission depends on the size, location, and type of extension. Many smaller extensions are exempt from planning permission under the Planning and Development Regulations 2001 (Schedule 2), but the rules have strict conditions.

When You Don't Need Planning Permission

Ground floor rear extensions are exempt if all of the following conditions are met:

Total floor area of all extensions (including any previous extensions) does not exceed 45 square metres — raised from 40 square metres on 27 July 2026 by SI 338/2026
The extension is to the rear of the house (behind the rear building line)
Private open space is not reduced below 25 square metres
The extension does not exceed the height of the existing house
Ground-level windows are at least 1 metre from the boundary they face

Above-ground extensions have tighter limits:

Detached houses: Up to 20 square metres above ground floor level
Semi-detached and terraced houses: Up to 12 square metres above ground floor level
Windows at above-ground level must be at least 8 metres from the boundary they face (reduced from 11 metres in July 2026)
The roof of the extension cannot be used as a balcony or terrace
The extension must be at least 2 metres from any boundary

These are cumulative limits — all previous extensions since 1 October 1964 (or the date the house was built, if later), including extensions that were granted planning permission, count towards the total. The exemption can only be used on the principal house — not on a Class 3A garden dwelling.

When You DO Need Planning Permission

You need planning permission for an extension if:

The extension exceeds 45 square metres (ground floor) or 20/12 square metres (above ground)
The extension is to the front or side of the house
The extension would exceed the height of the existing house
It would reduce private open space below 25 square metres
You are building a dormer window (always requires permission on any elevation)
Your property is a protected structure or in an Architectural Conservation Area (ACA)
The cumulative total of all extensions (including previous ones) exceeds the thresholds
The extension would create a traffic hazard

The Application Process

If your extension needs planning permission:

Consider a pre-planning consultation with your council (free)
Hire an architect to prepare drawings (site plan, floor plans, elevations)
Erect a site notice and publish a newspaper notice
Submit the application with all documents and pay the fee
The council decides within 8 weeks (see our timeline guide)

Fees

Application Type	Fee
Extension to house	€34
Retention of extension (built without permission)	€102 (3x standard)

See our full fees guide for the complete schedule.

Common Conditions Attached to Grants

If the council grants permission for your extension, common conditions include:

Materials and finishes must match the existing house
Drainage requirements (surface water management, connection to sewers)
Completion timeline (typically 5 years)
Compliance with building regulations
Protection of neighbouring amenity (e.g. no windows overlooking neighbours)
Hours of construction work

Party Wall Considerations

If your extension involves a shared (party) wall with a neighbour — common with semi-detached and terraced houses — be aware that:

Planning permission does not grant you the right to build on, attach to, or project across a party wall into your neighbour's property
The planning authority does not determine ownership of walls or boundaries — that is a civil matter
You should enter into a party wall agreement with your neighbour covering access arrangements, permitted work times, flashing and finish details, and the right to use the wall as a party structure
The Land and Conveyancing Law Reform Act 2009 contains provisions dealing with party wall matters
Discuss your plans with your neighbour early — this can avoid disputes, objections, and costly delays

Protected Structures and Conservation Areas

If your home is a protected structure (on the Record of Protected Structures) or in an Architectural Conservation Area (ACA):

The normal exempted development rules do not apply — virtually all external works require planning permission
You can request a Section 57 Declaration from your local authority (free) to establish what works would or would not materially affect the character of the building
Every local authority has an Architectural Conservation Officer who can advise on appropriate materials, design, and approach
See our Exempted Development guide for full details

Roof Windows and Dormers

Velux/roof windows on the rear slope are generally exempt (max 20% of roof area, max 15cm protrusion)
Velux on front or side elevations require planning permission
Dormer windows always require planning permission — they change the external appearance of the building

Building Regulations

Even if your extension is exempt from planning permission, it must still comply with building regulations. You may need:

A commencement notice to the Building Control Authority
Compliance with Part L (energy efficiency), Part B (fire safety), and other parts
An assigned certifier for certain works

Planning permission and building regulations are separate legal requirements — meeting one does not satisfy the other.

Related Guides

Exempted Development — all exemption thresholds and conditions
Do I Need Planning Permission? — general guide
How to Apply for Planning Permission — the full application process
Retention Planning Permission — if you've already built without permission

Disclaimer

This guide provides general information about planning permission for house extensions in Ireland based on the Planning and Development Regulations 2001 (as amended). Exemption rules are complex and have many conditions. Always verify with your local authority or a qualified planning professional before starting work.