How Much Does a 40m² Extension Cost in Ireland? 2026 Prices for 15 to 45 m²

Shrinking an extension does not shrink the bill in step. A 15 m² extension in Dublin can still cost up to €58,000, or €3,867 per m². Foundations, a steel opening, drainage and a kitchen cost much the same whatever the floor area. That is why the per m² price climbs as the extension gets smaller.

Key Findings for 2026

How Much Does a 40m² Extension Cost in Ireland?

A 40 m² single-storey rear extension costs €108,000 to €138,000 to build in Dublin in 2026. In the rest of Ireland, the same build costs €96,000 to €120,000. Both figures are Build Me estimates for a standard finish, including VAT at 13.5%.

The Dublin range uses a rate of €2,700 to €3,400 per m². At the top end, the minimum project cost for a 40 m² job lifts the figure from €136,000 to €138,000. Outside Dublin, the rate is €2,400 to €3,000 per m².

The build price is not the full budget, and the house extension cost guide breaks down every hidden cost. Professional fees for an architect, engineer and BER typically add 10 to 12% of the build cost. A 15% contingency covers what only appears once the ground is open. In Dublin, that brings a 40 m² project to about €135,000 to €175,300. A new kitchen and any steelwork come on top.

Forty square metres is a big room. It is roughly a full-width kitchen, dining and living space across the back of a typical semi. For many families, it replaces a move to a bigger house. Since July 2026, a rear extension of up to 45 m² can also skip planning permission, if it meets the conditions below.

How Much Does an Extension Cost by Size?

The table below gives single-storey rear extension prices for six common sizes. It shows the build cost and the total project budget, with fees and contingency included. Dublin rates run about 10 to 15% above the rest of Ireland, mainly on labour, access and waste removal (Build Me estimate).

15 to 20 m² Extensions

A 15 m² extension costs €40,500 to €58,000 in Dublin and €36,000 to €50,000 elsewhere. A 20 m² extension costs €54,000 to €69,000 in Dublin and €48,000 to €60,000 elsewhere. These are the sizes most often used to enlarge a galley kitchen or add a small dining area.

At this size, the minimum project cost does most of the work. In Dublin, no extension up to 15 m² is priced below €40,000, and the top of the range reaches €58,000. At the top, that is well above the standard €3,400 per m² rate.

The total budget for 20 m² in Dublin, with fees and contingency, is about €67,500 to €87,600. Small extensions still take most of the standard 12 to 16 weeks on site. The digging, the slab, the roof and the opening into the house all take time, whatever the size.

25 to 30 m² Extensions

A 25 m² extension costs €67,500 to €98,000 in Dublin and €60,000 to €85,000 elsewhere. A 30 m² extension costs €81,000 to €102,000 in Dublin and €72,000 to €90,000 elsewhere. This band suits a proper kitchen-diner across most of the back of a semi.

The 25 m² range is unusually wide. That is because the minimum project cost for jobs up to 30 m² is €58,000 to €98,000 in Dublin. At 25 m², the €98,000 ceiling sits above the €85,000 you would get at €3,400 per m².

By 30 m², the standard per m² rate takes over in full: €2,700 to €3,400 per m² in Dublin. The total budget for 30 m² in Dublin is about €101,300 to €129,500, before any kitchen.

40 to 45 m² Extensions

A 40 m² extension costs €108,000 to €138,000 in Dublin and €96,000 to €120,000 elsewhere. A 45 m² extension costs €121,500 to €153,000 in Dublin and €108,000 to €135,000 elsewhere. These are the largest single-storey rear extensions you can usually build without planning permission.

The extra 5 m² from 40 m² to 45 m² adds about €13,500 to €15,000 to the Dublin build cost. That is close to the plain per m² rate, because the fixed costs are already paid. It is often the cheapest floor space in the whole project.

With fees and contingency, a 45 m² extension in Dublin needs a total budget of about €151,900 to €194,300. Outside Dublin, budget €135,000 to €171,500. Large single-storey extensions still take 12 to 16 weeks on site. Most trades work on the whole floor at once.

Extension Cost Calculator by Size

Move the slider to your floor area and pick your location. The calculator uses the same standard-finish rates and minimum project costs as the tables above. Tick the extras if your project includes a new kitchen or a steel opening into the house.

The calculator covers single-storey rear extensions only. For side, wrap-around or glazed builds and different finish levels, the full extension cost calculator covers every type.

Why Do Small Extensions Cost More per m²?

Every extension carries costs that barely change with size. Here is why. A 15 m² and a 45 m² extension both need a site set-up and inspected foundations. Both need a new drain connection and a roof tied into the house. Both need an opening knocked through the old back wall, usually with a steel beam.

Those fixed items are spread over fewer square metres on a small job. Moderate structural work costs €3,000 to €8,000, and complex work €8,000 to €20,000. A new kitchen costs €15,000 to €40,000 whether the room is 15 m² or 45 m². Design fees also have a floor, because the drawings and engineer's calculations take similar time.

The result is clear in the chart below. With a new kitchen and a steel opening added, a 15 m² Dublin extension costs €4,580 to €8,110 per m² all in. At 45 m², the same package costs €3,780 to €5,380 per m². The small job costs up to 50% more for each square metre.

In practice, this means the first few square metres are the dearest. Going from 30 m² to 40 m² in Dublin adds about €27,000 to €36,000 to the build cost. That is far less per m² than the first 15 m² cost. If your garden allows it, building the size you need once is usually cheaper than extending twice.

How Does a Two-Storey Extension Compare per m²?

A two-storey rear extension costs less per m² than a single-storey one. In Dublin, budget €2,200 to €3,100 per m², against €2,700 to €3,400 for single-storey. In the rest of Ireland, the two-storey rate is €1,900 to €2,700 per m². The two floors share one set of foundations and one roof.

The catch is planning. The 2026 exemption allows only 12 m² upstairs on a terraced or semi-detached house, or 20 m² on a detached one. Any first-floor extension must also be at least 2 m from any party boundary. Most two-storey house extensions therefore need planning permission, which takes 12 to 16 weeks from preparing drawings to a decision.

By comparison, at the top of the range a 40 m² two-storey build costs the same as single-storey. The saving shows at the low end and on bigger builds. Two-storey also keeps more of the garden, which matters where the 25 m² open space rule is tight.

What Size Extension Can You Build Without Planning Permission?

Since 27 July 2026, a rear extension of up to 45 m² can usually be built without planning permission. The limit was 40 m² before. The change comes from the 2026 exempted development regulations (S.I. 338 to 344 of 2026). The 45 m² counts every extension built since 1 October 1964, including ones that had planning permission.

So if your house already has a 12 m² kitchen extension from the 1980s, you have 33 m² left. A garage conversion at the side or rear also counts towards the same 45 m². Side and front extensions are not covered and always need planning permission.

Exemptions do not apply to protected structures, or where an earlier planning condition removed them. If you are unsure, a Section 5 declaration from your council confirms it. It costs €80 and normally takes about 4 weeks.

Building Regulations still apply in full, for structure, fire, ventilation, energy and access. A Commencement Notice is not needed for a house extension of up to 45 m² in total (S.I. 359 of 2026, also up from 40 m²).

Worked Example: What Does a 40 m² Kitchen Extension Cost in Dublin?

This is a worked example, not a Build Me project. It shows how the figures in this guide add up for a common job. The house is a 1950s semi in south Dublin that has never been extended. The owners want a 40 m² single-storey rear extension for a new kitchen, dining and living space.

The design opens up the old back wall, so it needs a moderate steel opening. The rear garden is 14 m long, so plenty of open space remains. The extension is under 45 m² and to the rear, so it needs no planning permission.

The kitchen is the biggest swing in the budget. A €15,000 kitchen versus a €40,000 one moves the total by €25,000. The finish level of the extension itself, from floors to glazing, explains most of the remaining gap. The layout of most kitchen extensions follows the sink and drains, so fix the kitchen plan before the build starts.

On time, plan for 12 to 16 weeks on site. Allow extra time before that for design drawings and quotes. Build Me builds single-storey extensions in traditional block, which is the construction priced throughout this guide.

How Do You Budget for Your Extension Size, Step by Step?

Size is the first decision, and it drives everything after it. These steps take you from a rough idea to a budget you can take to builders.

1. Week 1Measure the space you need.List the rooms or furniture the extension must hold and mark the footprint in the garden with pegs and string.
2. Week 1Check what you have already built.Add up every extension since 1 October 1964, including garage conversions, to see how much of the 45 m² exemption is left.
3. Week 2Price your size with the per m² rate.Multiply the floor area by €2,700 to €3,400 in Dublin, or €2,400 to €3,000 elsewhere, and apply the minimum project cost for 40 m² or less.
4. Week 2Add fees, contingency and extras.Add 10 to 12% for professional fees, 15% contingency, a kitchen at €15,000 to €40,000 and any structural work.
5. Weeks 3 to 8Get drawings and written quotes.Have an architect or engineer draw the design, then get fixed quotes from builders who have visited the site.

Will Extension Costs by Size Rise in 2027?

Prices are likely to keep rising slowly rather than jump. These trends are tied to current Irish data.

• Concrete leads materials inflation: CSO data for August 2026 shows ready mixed mortar and concrete up 7.6% in a year. Foundations and slabs weigh most heavily on small extensions, so their per m² price may rise fastest.
• Steel costs keep climbing: the same CSO release shows reinforcing metal up 10.8% and structural steel up 4.1% year on year. That feeds straight into the €3,000 to €8,000 cost of a steel opening.
• Overall build costs drift up: the CSO Building and Construction Index (materials and wages) rose 3.3% in the year to August 2026. Expect per m² extension rates to rise by a similar 3 to 4% into 2027 (Build Me estimate).
• More 41 to 45 m² extensions: the new 45 m² limit lets many homeowners add 5 m² without planning. Expect more designs sized to the new limit, and more homeowners weighing whether to extend or move.

FAQ

Next Steps

1. This week: measure your garden and mark out the size you want, leaving at least 25 m² of open space.
2. Within 1 week: check your house deeds or council records for any extension built since 1964.
3. Within 2 weeks: price your size with the calculator above and decide whether you need a new kitchen.
4. Within 1 to 2 months: get drawings and at least three written quotes from builders who have seen the site.
5. Before work starts: confirm the exemption, or apply for planning permission if you are over 45 m² or building to the side.

Useful Tools and Resources

• South Dublin County Council, new exemptions from 27 July 2026: a plain summary of the 45 m² extension limit and other changes.
• S.I. 338 of 2026 (full text): the exact Class 1 conditions for rear extensions.
• Department of Housing circular PL 02/2026: the official circular on the new exempted development rules.
• CSO Wholesale Price Index: monthly building material price changes in Ireland.

How We Checked These Figures

All per m² rates, minimum project costs, fees, contingency, kitchen and structural figures come from the Build Me extension calculator, checked on 25 October 2026. They are estimates for a standard finish, including VAT at 13.5%, not quotes. Size totals multiply the rate by the floor area and apply the minimum project cost for 40 m² or less.

Planning rules were checked against the text of S.I. 338 of 2026 and council summaries. Material price trends come from the CSO Wholesale Price Index for August 2026. Where sources disagreed, we used the regulation text. We re-check these figures every quarter.

Sources

1. Planning and Development (Exempted Development (Act of 2000)) Regulations 2026, S.I. 338 of 2026, signed 16 July 2026 (accessed 25 October 2026)
2. South Dublin County Council, New exemptions from 27 July 2026 (accessed 25 October 2026)
3. Galway City Council, Exempted Development: Department of Housing Planning Circular Letter PL 02/2026 (2026)
4. Central Statistics Office, Wholesale Price Index August 2026 (published 22 September 2026)