Do you need planning permission for a house extension in Northern Ireland?

Many homeowners in Northern Ireland can extend their house without making a planning application, provided the work falls within “permitted development” rights. Those rights are set out in law. They are subject to detailed size, height, siting and design limits, and they can be restricted or removed.

If you build without the permission you need, nidirect is clear that the council may force you to put things right later. The safe approach is to check the proposal against the current rules, and to ask your local planning office before work starts.

This article summarises the main tests for a typical house extension. It is general information, not a formal determination.

Permitted development is a legal test, not a rule of thumb

The principal source is The Planning (General Permitted Development) Order (Northern Ireland) 2015 (the GPDO). Class A of Part 1 covers the enlargement, improvement or other alteration of a dwellinghouse. The Order has been amended on several occasions, including as recently as 13 May 2026, so the current official text should always be checked.

nidirect explains that certain works can be carried out under permitted development rights, but only if specified conditions are met, and advises you to contact the council to check. Council guides, including Mid and East Antrim Borough Council’s Your Home and Planning Permission, set out the same limits in plainer language. The Order itself is the legal source.

The main limits for a house extension

The following is a summary of important Class A limits. All of them can matter, not only the rearward projection. Measurements should be taken externally. Development is not permitted by Class A if, among other things:

• as a result of the works, buildings within the curtilage (other than the original dwellinghouse) would cover more than 50 per cent of the curtilage, excluding the ground area of the original dwellinghouse
• the enlarged part would be higher than the highest part of the existing roof, or its eaves would be higher than the existing eaves
• the enlarged part would extend beyond a wall which faces onto a road and forms the principal or a side elevation of the original dwellinghouse
• a single-storey enlargement would extend more than 4 metres beyond the rear wall of the original dwellinghouse if the house is detached, or more than 3 metres in any other case; would exceed 4 metres in height; or would be within 3.5 metres of the boundary with a road opposite the rear wall
• an enlargement of more than one storey would extend more than 3 metres beyond the rear wall of the original dwellinghouse, or would be within 7 metres of the boundary opposite the rear wall
• the enlargement would be within 2 metres of any boundary and the eaves of the enlarged part would exceed 3 metres in height
• a side extension would exceed 4 metres in height, or would be wider than half the width of the original dwellinghouse

Class A also does not permit an alteration to any part of the roof, a deck, balcony, veranda or other raised platform, or a basement. Roof alterations and porches have separate classes, with their own limits.

Conditions also apply. Exterior materials, other than those used in a conservatory, must be of similar appearance to the existing house. Certain upper-floor side windows within 15 metres of a neighbouring dwellinghouse boundary must be obscure glazed and non-opening unless the opening parts are more than 1.7 metres above floor level. Where the enlargement has more than one storey, the roof pitch should, as far as practicable, match that of the original dwellinghouse.

Many limits are measured from the original dwellinghouse, not simply from the house as it stands after earlier extensions. The precise legal meaning of that term is set out in the Order and should be checked with the council. A previous planning permission may also have removed or limited permitted development rights. nidirect gives the example of a condition preventing a front boundary wall or fence on an open-plan estate.

When you should expect to need planning permission

You should not assume permitted development applies if the extension would exceed any of the limits above, or if the property is a flat or maisonette (nidirect states that permission is needed to extend a flat or maisonette); if you would create a separate home in the house or garden; if the house is listed, or the site is in a conservation area or an Area of Outstanding Natural Beauty; or if a planning condition has already limited what you can build. nidirect states that permitted development rights are restricted in those designated areas, and the GPDO also tightens Class A in conservation areas, World Heritage Sites, Areas of Outstanding Natural Beauty and National Parks. Listed building consent is a separate process from planning permission.

Garden rooms and other approvals

A detached garden room, garage or shed is generally considered under Class D of the same Order, not as a house extension. Class D permits certain buildings required for a purpose incidental to the enjoyment of the dwellinghouse, subject to height, siting and coverage limits, including a 4-metre height limit and a 2.5-metre eaves limit within 2 metres of a boundary. It does not permit development for use as a dwellinghouse. If the building would be used as a separate home, or for a use that is no longer incidental to the house, planning permission is likely to be required.

Even where planning permission is not required, other consents may be. nidirect notes that title covenants or lease restrictions can require someone else’s agreement. Building regulations are also separate from planning; see the nidirect building regulations pages and confirm the position with your council’s building control service. For a formal decision on whether a proposal is lawful, nidirect explains that you can apply to the council for a Certificate of Lawful Use or Development. A fee is payable.

Frequently asked questions

Can I always build a 4-metre rear extension on a detached house without permission?

No. The 4-metre figure is only one of several Class A tests, and it is measured from the original dwellinghouse. Height, coverage, boundaries, designated-area rules and any planning conditions also apply.

What if I only want a small porch?

Porches have a separate class, including a 3-square-metre ground area limit measured externally. Check Class C of the Order, or ask the council.

Do I need permission to replace windows or carry out internal alterations?

Internal alterations that do not change the use of the house, and window or door replacements in existing openings, often do not need planning permission, but listed buildings and conservation areas are different. Building regulations may still apply.

Can you guarantee that my extension is permitted development?

No. Only the council can issue a formal certificate of lawfulness. We can review the proposal and advise on the safest next step.

This article is for general information only. It is not formal planning advice or a certificate of lawfulness, and it does not guarantee any particular outcome. Always check the current position with the relevant council planning office before starting work.

Official sources

• The Planning (General Permitted Development) Order (Northern Ireland) 2015 (current schedule)
• Planning permission – when to apply (nidirect)
• Planning permission – before you start work (nidirect)
• Getting help with planning, including Certificates of Lawful Use or Development (nidirect)
• Building regulations (nidirect)
• Planning offices in Northern Ireland (nidirect)
• Mid and East Antrim Borough Council, Your Home and Planning Permission – extensions
• Mid and East Antrim Borough Council, Your Home and Planning Permission – outbuildings

Free guide: download our Extension checklist at https://niplanningpermission.co.uk/free-planning-guides/