House extension questions, answered

Building on one level costs €1,500 to €2,200 per m² across Ireland, and building two storeys €2,000 to €2,500 per m² with both floors counted, VAT at 13.5% included. Dublin and the commuter counties sit higher. With design and engineer's fees, a 20 m² ground-floor room comes to €42,000 to €54,000 and a 40 m² two-storey extension with an en-suite to €105,000 to €128,000.

Per m² across Ireland, VAT included: one storey €1,500 to €2,200, two storeys €2,000 to €2,500, a side extension €1,800 to €2,400, a wraparound €2,200 to €3,200 and mostly glass €2,800 to €3,500. Every line is higher in Dublin.

€15,000 to €25,000 at a standard finish, €20,000 to €30,000 mid-range, €30,000 to €45,000 high spec or with an en-suite.

€4,000 to €8,000 to fit out with standard fittings, €10,000 to €18,000 mid-range, €20,000 to €40,000 high-end, inside the new room.

Since 27 July 2026 you can build up to 45 m² behind the house without applying for planning permission. That 45 m² includes every extension added since 1964 and any attached garage you convert. The extension can't rise above the house, the house has to keep 25 m² of private open space, ground-floor windows need 1 m or more to the boundary they look onto, and no more than 12 m² of it can be upstairs on a semi or terrace (20 m² on a detached house). Before that date the limit was 40 m².

On 27 July 2026 the exempt area behind the house rose from 40 m² to 45 m², the distance from an upstairs window to the boundary came down from 11 m to 8 m, and the building control threshold moved to 45 m².

Nearly always. Only 12 m² upstairs is exempt on a semi or terrace, 20 m² on a detached house.

Yes, unless it is the conversion of an attached garage, store or shed at the side.

For an extension over 45 m², or one with planning permission: lodge it 14 to 28 days before work starts. An exempt extension of 45 m² or less needs none.

Design and pricing take 8 to 16 weeks. Building then takes 8 to 14 weeks for one storey and 14 to 24 weeks for two.

13.5% on the building work and on fixtures the builder supplies and fits. 23% on appliances, professional fees, and fixtures you buy yourself.

Not a general one. A council can pay up to €40,000 under the Housing Adaptation Grant for an extension that a person with a disability needs.

Moving costs money that buys no extra room. On a €380,000 house, the median people moving home paid in 2025, stamp duty on the next house is €3,800, the agent's fee on the sale is €4,700 to €11,700 with VAT and the solicitor €2,000 to €5,000: €10,500 to €20,500 before removals, and before the higher price of a bigger house. Extending keeps all of it in the house you already own.

Give every builder one set of drawings, and ask for the steel, the glazing, any en-suite, the fees and the VAT on lines of their own.

We price the plan and pass it to a single vetted builder for your area.

A single vetted builder for your area receives your answers with our estimate and calls to set up a visit. The written price comes after that, and it's yours to accept or turn down.

Under Schedule 2, Part 1, Class 1 of the Planning and Development Regulations (from 27 July 2026), converting an attached domestic garage to the side or rear of a house for use as part of the house is included in the 45 m² cumulative floor area allowance. If you convert a 15 m² attached garage and also build a rear extension without planning permission, the combined floor area of both cannot exceed 45 m².

A single-storey rear extension can be built right up to the side boundary line provided no part of the foundation, wall, fascia or rainwater gutter overhangs the boundary into your neighbour's side (builders use an internal box parapet gutter along the boundary wall), and ground-floor windows facing a boundary sit at least 1 metre back. For an exempted two-storey rear extension, the upper storey must be set back at least 2 metres from any party boundary.

A builders' finish quote covers everything up to a plastered, watertight shell with screed floors, glazed doors and windows, radiators and white sockets, leaving you to buy and fit your own kitchen, floor tiles and paint. A turnkey quote includes the finished flooring, tiling, sanitaryware and decoration.

Almost every Irish estate house has a 100 mm or 150 mm foul sewer and storm pipe running across the back garden 1 to 2 metres behind the original back wall. Your builder will open the existing inspection chamber at survey stage: any manhole must be moved outside the new extension footprint, and pipes passing through foundation trenches are bridged with concrete lintels and flexible rocker pipes.

Even when planning permission is not required, you still need working drawings, a structural engineer's design for the steel beam opening up the back wall, and an Opinion on Compliance with Planning and Building Regulations at the end of the job. Without that certificate, a future buyer's solicitor will hold up the sale of your house until retrospective sign-off is arranged.

Never pay a large upfront cash deposit before work starts on site. A standard Irish extension payment schedule runs in five stages inspected after the work is in place: 1) Substructure up to insulated floor slab, 2) Superstructure walls and roof timber or steel, 3) Roof weathered and external glazing fitted, 4) First-fix plumbing, electrics, insulation and plastering, and 5) Practical completion less a 2.5% to 5% snagging retention.