Chapter 9 of 10 · Self-build, extending and renovating · 4 of 8
House extension costs: what a square metre costs, the 40 square metre rule, and how to pay for it
Checked 21 Oct 2026the Central Bank's mortgage measures, Revenue's rules and the scheme operators' published terms.
Typical extension costs per square metre, the 40m² exemption, architect and engineer fees, finance and SEAI grants you can combine.
Check before you hire: a architect should be on the register kept by the RIAI. RIAI: register of architects
Written in plain English for the person buying, selling or switching. This guide covers the Republic of Ireland only.
An extension is the most common building project in Ireland. It is also the one where people most often run out of money halfway. The reason is that the quoted figure is for the shell, and the shell is only part of what you pay. This guide gives typical costs per square metre, lists everything that comes on top, explains the 40 square metre planning exemption, and sets out how people pay for it.
What a square metre costs
Builders quote extensions by floor area. The figure covers the structure, the roof, the windows and doors, the plastering, the electrics and plumbing, and a basic finish. It does not usually cover the kitchen, the flooring, the fees, or any work to the existing house.
| Type | Typical range per square metre (2026, including VAT at 13.5%) |
|---|---|
| Single-storey rear extension, standard finish | About €2,500 to €4,000 |
| Two-storey extension | A little less per square metre, because one roof and one set of foundations serve two floors |
| High finish, glazing, structural steel, Dublin | Towards the top of the range and above |
These are typical ranges from published cost guides, including the SCSI's, and from quantity surveyors; they are not quotes. Costs vary with the site, the ground, the design and the year. Get a quantity surveyor (the SCSI) to cost your drawings, or three builders to tender on them. The extension costs for a derelict or older home are a different kind of job and are covered separately.
A worked example
Fiona and Mark want a 30 square metre kitchen extension on the back of a semi-detached house.
| Item | Amount |
|---|---|
| Build at €3,000 a square metre | €90,000 |
| Architect's fee, planning drawings to completion | €8,000 plus VAT |
| Engineer's structural design and certificates | €2,000 plus VAT |
| Kitchen and appliances | €15,000 |
| Flooring and tiling | €4,000 |
| Knocking through and making good the existing house | €6,000 |
| Contingency at 10% of the build | €9,000 |
| Total | About €136,000 |
The €90,000 headline became €136,000. That is normal. The architect page explains what the fee covers; a full service typically runs from €2,500 for drawings alone to €15,000 or more plus VAT for a project managed to completion.
What comes on top of the build cost
- Design fees. An architect (the RIAI register) or an architectural technologist draws it, lodges planning if needed, and can manage the build.
- Structural fees. An engineer (Engineers Ireland; see the structural survey page) designs the steel beam where you knock through, and signs the certificate of compliance at the end.
- The planning application, if one is needed: the council's fee, the notices, and the drawings. The planning permission guide lists them.
- The kitchen, bathroom, flooring, tiling and lighting.
- Work to the existing house: new openings, moving the boiler, rerouting drains, matching the roof.
- Decorating, the garden, the patio.
- A contingency of 10%, because every extension finds something in the ground or in the walls.
- Living costs if you move out during the work.
The 40 square metre rule
A rear extension does not need planning permission if it meets every one of these conditions:
- The total floor area, counting any extension built since 1 October 1964, is no more than 40 square metres.
- At least 25 square metres of private open space is left at the rear.
- For a two-storey extension, the part above ground level is no more than 12 square metres on a terraced or semi-detached house, or 20 square metres on a detached house, within the 40.
- The height does not exceed the limits for the roof type, and the extension is no higher than the existing house.
- Windows at ground level are at least 1 metre from the boundary, and at upper level at least 11 metres from the boundary they face.
- The house is not a protected structure and is not in an architectural conservation area with stricter rules.
A 30 square metre extension on a house with an earlier 15 square metre one needs permission, because the total is 45. Anything to the front or side needs permission in nearly every case. If you are unsure, ask the council for a section 5 declaration, which gives a written answer you can show a buyer later. Even an exempt extension must meet the building regulations, and a lender or a buyer's solicitor will ask for a certificate of compliance from your engineer.
How people pay for it
| Option | How it works | What to know |
|---|---|---|
| Top-up on your mortgage | Your lender adds to your loan | Usually the lowest rate; the lender values the house and tests your income again; runs for the rest of the term |
| Remortgage to a new lender | A new, bigger mortgage with another lender | Outside the Central Bank limits if it is a switch; a solicitor and a valuation are needed |
| Green top-up | A top-up at a lower rate for works that bring the BER to B3 or better | The whole house, not just the extension, has to reach the rating |
| Credit union or personal loan | A separate loan over 5 to 10 years | Higher rate, paid off sooner, no charge on the house |
| Savings | No borrowing | Keep a contingency |
A top-up is a mortgage, and the Central Bank's rules on top-ups for home improvement allow lenders some room, but each lender applies its own limits. The top-up guide covers the lender rules and the green top-ups. Put the top-up amount into the repayment calculator to see what it adds each month. On €100,000 over 20 years at 3.5%, that is about €580 a month.
SEAI grants alongside
SEAI does not grant-aid an extension. It does grant-aid energy upgrades to the existing house, and an extension is the sensible time to do them, because the builder is there and the walls are open. The grants cover attic and wall insulation, windows and doors in some cases, a heat pump, and solar panels, with a one-stop-shop route that bundles them. Most need a registered contractor and a BER afterwards. The SEAI grants guide lists them and the BER guide explains the rating.
The order of the work
- Decide what you need the space for and what you can spend, including the items on top.
- Appoint a designer and get sketches.
- Check the planning position: exempt, or an application.
- Get the structural design.
- Cost the drawings with a quantity surveyor, or tender to three builders.
- Sign a building contract with a fixed price and a programme.
- Arrange the finance and the insurance; tell your home insurer the work is happening.
- Lodge a commencement notice with the council at least 14 days before starting, where one is needed.
- Build, with stage payments against the engineer's inspections.
- Get the certificate of compliance, the BER if you changed the heating or insulation, and keep every document with the deeds.
Where this leaves you
Put the top-up you need into the repayment calculator and see what it adds to the monthly figure, then follow the self-build journey for the order of the professionals. An architect or engineer who costs the job before you commit is the single biggest saving; get quotes before you talk to a builder.
Questions people ask
How much does a house extension cost per square metre in Ireland?
Typical quotes in 2026 run from about €2,500 to €4,000 a square metre for a standard single-storey extension, including VAT at 13.5%, with Dublin at the top of the range and a two-storey extension cheaper per square metre. Fees, the kitchen and finishes come on top.
Do I need planning permission for a 40 square metre extension?
A rear extension of up to 40 square metres is exempt if the house has not been extended since 1964, at least 25 square metres of garden is left, and the height and window rules are met. Earlier extensions count towards the 40. To the front or side, you need permission.
How can I finance a house extension?
A top-up on your mortgage, a remortgage to a new lender, a green top-up if the works improve the BER, a credit union or personal loan, or savings. A top-up is usually the lowest rate but adds to a loan that runs for decades.
Can I get an SEAI grant for an extension?
Not for the extension itself, but for energy upgrades to the existing house done at the same time: attic and wall insulation, windows, a heat pump, solar panels. Most need a BER assessment afterwards, and a registered contractor.
How long does an extension take to build?
A single-storey rear extension typically takes three to five months on site. Add two to four months before that for design, the planning decision if needed, and tendering. Living in the house during the work is possible but disruptive.