Planning permission can feel like one of those things where everyone has a slightly different version of what the rules actually are. Your neighbour tells you they built a big rear extension without any bother. Someone in a Facebook group says you absolutely need permission for anything over a certain size. Your builder shrugs and says it depends. The truth is more specific than any of that, and getting it right at the start saves you real hassle later.
Do I always need planning permission to extend my home?
No. In Ireland, many residential extensions fall under what is called exempted development, which means you can build without applying to your local council, provided you stay within certain limits and conditions. The key word there is "conditions". Exempted development is not a free pass to build whatever you like. It is a defined set of rules, and if you stray outside them, even accidentally, you are in planning breach territory.
What are the actual size limits for a rear extension?
For a house (as opposed to an apartment or flat), the general rule is that a rear extension can be exempted if the floor area of the extension, combined with any previous extensions added since 1963, does not exceed 40 square metres. That figure applies to houses in general. For terraced houses and semi-detached houses, the limit is 40 square metres total. For detached houses, it is also 40 square metres, though some older guidance distinguished between these categories. The important thing to check is the cumulative figure. If a previous owner added a 20 square metre extension ten years ago, you only have 20 square metres of exempted headroom left, not 40.
This is the detail that catches a lot of people out, particularly on older properties that have already been extended once.
Are there height restrictions too?
Yes. The height of the extension matters, not just the footprint. A single-storey rear extension must not exceed the height of the existing house. That sounds straightforward, but it catches people who want to add a flat roof with a rooflight or a parapet that pushes above the original eaves line. If you are weighing up roof types, the differences between flat roof and pitched roof extensions are worth understanding before you commit to a design, because the choice affects what stays within exempted limits.
What about building on the side or to the front?
Side extensions are treated differently and the rules are tighter. Any side extension must not reduce the side passage width to less than one metre. Front extensions are generally not exempted at all. If you want to add a porch to the front, there is a separate and smaller exemption that applies specifically to porches, but it has its own conditions around size and distance from the road.
What conditions apply beyond size?
Several. The extension must not reduce the open garden area behind the house to less than 25 square metres. The roof of the extension must not be used as a balcony or roof terrace. Any upstairs window in a side wall must be a minimum of 1.7 metres above floor level, to prevent overlooking neighbours. Windows in rear walls must not directly overlook adjoining properties to an unreasonable degree. The materials used should generally match the existing house, though this is one of those conditions that is easier to assess in practice than on paper.
All of these conditions need to be met simultaneously. Hitting the size limit but failing on the garden area, for example, means the entire project falls outside the exemption.
Does exempted development mean I do not need any approval at all?
It means you do not need planning permission. It does not mean the build is unregulated. Building Regulations still apply. You will need to comply with structural, fire safety, insulation and ventilation requirements regardless of whether permission is required. Your builder or architect will need to submit a Commencement Notice to your local authority before work starts, and a Certificate of Compliance on Completion is something you will want at the end, not just for your own peace of mind but because solicitors will ask for it when you come to sell.
How do I know if my specific site has any restrictions on exempted development?
A few situations remove exempted development rights entirely. If your house is a protected structure, or is within an Architectural Conservation Area, the normal thresholds do not apply. Some planning permissions granted in the past, especially on newer estates, include conditions that specifically remove certain exemptions. Your original planning file is public and can be checked with your local council. If there is any doubt, a quick call to the planning department or a short consultation with an architect who knows local practice is money well spent.
For a clear and reliable summary of the full rules as they stand, the Citizens Information page on planning permission for homeowners lays out the exempted development thresholds and conditions without the legal language.
The best approach before drawing up any plans is to check the planning history of your property, measure what is already there, and only then work out what you actually have available. Getting a quantity surveyor's report reviewed early can also help you understand the full scope of what you are committing to before a shovel goes in the ground.