Side extensions
A side extension builds on the strip beside the house, over the side passage or a drive, on one floor or two. One storey at the side runs at €1,800 to €2,400 per m² across Ireland and €2,000 to €2,700 per m² in Dublin, VAT included. It always needs planning permission: the exemption covers building behind the house and converting an attached garage, not new building at the side.
Building on the side of the house: what it costs, what it gives a semi-detached or detached house, and the planning it needs.
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- €1,800–€2,400per m² for one storey, VAT in
- €2,000–€2,700per m², Dublin
- €34council fee to apply
- 12 weeksdecision and appeal period
What a side extension gives you
The side of a semi-detached or detached house is often the least used strip of ground it has: a passage for the bins, or a driveway that runs past the house. Built on, it becomes a utility and a downstairs toilet, a study, a playroom, or a wider kitchen. Taken up two floors, it adds a bedroom over the top.
On a corner site or a wide plot it can be the way to extend without giving up the back garden, where 25 m² of private open space has to remain under the exemption.
What it costs
€1,800 to €2,400 per m² (Dublin €2,000 to €2,700) on one floor, more than a plain rear extension. The site is narrow, materials come in by hand, the walls often run along a boundary, and the front of the house changes, so the design has to suit the street.
- A 15 m² single-storey side extension in Cork, mid-range: €36,000 to €45,000 all in, VAT and fees included.
- A 15 m² single-storey side extension in Dublin, mid-range: €41,000 to €51,000 all in, VAT and fees included.
- A 30 m² two-storey side extension at national rates, mid-range, with an en-suite upstairs: €83,000 to €103,000 all in, VAT and fees included.
A two-storey side extension is priced at the two-storey rate, €2,000 to €2,500 per m² across both floors.
Plan your house extension
Four quick taps, and your plan fills in as you answer: what it costs, whether it needs planning permission and how long it takes. Contact details come last, and only if you want a builder to price it.
15 m² side extension, mid-range finish: €37,000 to €46,000, including VAT and fees.
How the estimate is worked out
Every kind of extension has a build rate per m² for each region of Ireland, VAT at 13.5% included. A standard finish uses the lower half of that range and mid-range the upper half; high spec or a wall of glass moves the build to the glazed rate. Design, engineer and BER fees go on top, with anything you tick (an en-suite, a kitchen, a steel beam into the house) priced at your finish. A garage conversion gets one price for the whole job, since the walls and roof are already standing. Totals are rounded to the nearest €1,000. Use it to plan with. It isn't a quote: a builder prices your house after seeing it.
Planning permission
Class 1 covers extensions to the rear and the conversion of a garage, store or shed attached at the side or back. A new side extension isn't either, so it needs planning permission, whatever its size. Apply to the council with drawings and the newspaper and site notices: the fee is €34, the decision takes 8 weeks and there are 4 weeks for an appeal.
If there is a garage at the side already, converting it is exempt within the 45 m². Many side extensions start that way: the garage becomes a room, then a second storey goes on later with permission. Garage conversions.
Boundaries and the street
Councils look at how a side extension sits with the house and its neighbours: often a set-back from the front wall, a lower roof line, or a gap to the boundary. Where you plan to build on or against a wall you share, agree it with your neighbour before the builder touches it.
Other ways to extend
Each has its own page: costs by size, the planning rules that apply and the weeks of building.
Single-storey rearOne new room behind the house at ground level: a family room, a playroom or a bedroom for a parent.
Two-storeyTwo new floors in one build: more living space downstairs and an extra bedroom over it.
WraparoundAn L round the back corner of the house, for one big room with a utility or toilet off it.
Garage conversionThe garage you already have becomes a bedroom, an office or a playroom.
Bedroom and en-suiteOne more bedroom, downstairs or up, with a shower room of its own.Questions before you build
What does a side extension cost in Ireland?
A side extension builds on the strip beside the house, over the side passage or a drive, on one floor or two. One storey at the side runs at €1,800 to €2,400 per m² across Ireland and €2,000 to €2,700 per m² in Dublin, VAT included. It always needs planning permission: the exemption covers building behind the house and converting an attached garage, not new building at the side.
Does a side extension need planning permission?
Yes. The exemption covers building behind the house and converting attached garages, sheds and stores, not new building at the side.
Can I convert my side garage without planning permission?
Usually, yes: it is exempt under Class 1 if it is attached to the house, and it counts towards the 45 m² with any other extensions since 1964.
Can a side extension be two storeys?
Yes, with planning permission. It is priced at the two-storey rate, €2,000 to €2,500 per m² across both floors.
What can a side extension be used for?
A utility and downstairs toilet, a study or playroom, a wider kitchen, and a bedroom above if it goes up two floors.
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