House Extension Cost Ireland 2026: What You'll Really Pay

A house extension in Ireland in 2026 will cost between €2,100 and €3,900 per square metre, depending on the quality of finish and where in the country you're building. That's the headline. For a typical 30m² rear extension at mid-range spec, the total cost — including VAT, professional fees, and the smaller items most quotes leave out — lands between €115,000 and €130,000 in Dublin.

This guide breaks down exactly what an extension costs in Ireland in 2026 — by quality level, by county, and by all the line items that catch homeowners out after the builder's quote is signed. Figures are based on the SCSI House Rebuilding Guide (2024/2025) and the Buildcost Construction Cost Guide (H2 2025) — the same sources your architect and quantity surveyor will use.

Before you go any further, you can get a free estimate with the build cost calculator — it takes 60 seconds and gives you a county-specific extension cost range.

How Much Does a House Extension Cost in Ireland in 2026?

The headline figure is the cost per square metre. Here's where extension rates sit in 2026, by quality level:

For a typical 30m² rear extension at mid-range spec, that puts your base construction cost at approximately €75,000 – €93,000 before professional fees, VAT, and the smaller items.

By the time you account for everything a real extension in Ireland involves, the full project cost is typically 20–30% higher than the construction cost alone. Below is where every euro goes.

The Full Cost Breakdown

1. Construction Cost

The main spend. Covers groundworks and foundations, structural shell, roof, windows, external doors, plastering, plumbing, electrics, kitchen and bathroom fit, finishes, and the work to tie the new extension into the existing house. Roughly:

• Materials: ~45% of construction cost
• Labour: ~40% of construction cost
• Specialist subcontractors: ~15% of construction cost (electrical, plumbing, glazing)

For a 30m² mid-range rear extension in Dublin, base construction cost is approximately €84,000 (30m² × €2,800/m²).

2. Professional Fees

Most extensions need an architect or architectural technologist to draw up plans and submit any planning application. Architect fees on extension work typically run 8–12% of construction cost depending on complexity. On an €84,000 build, that's €6,700 – €10,100.

A structural engineer is almost always required for calculations on the steel beam at the new opening between the extension and the existing house. Budget €1,500 – €3,000.

3. Planning & Regulatory Fees

Whether you need planning permission depends on what you're building:

• A single-storey rear extension under 40m² to a non-protected house is usually exempted development and needs no planning permission, provided it meets every condition (eaves height, open space behind house, not a protected structure, not in an Architectural Conservation Area).
• Anything over 40m² to the rear, any extension to the side or front, any two-storey work, anything affecting a protected structure, and anything in an Architectural Conservation Area will require planning permission.

If planning is required:

Total planning and admin costs typically come to €800 – €2,500 for an extension that requires planning.

4. Services and Site Costs

These are the line items that almost never appear in the headline quote but always appear on the final invoice:

Total services and site costs typically add €5,000 – €15,000 depending on what your site throws up.

5. VAT

Construction services in Ireland attract VAT at 13.5% — the reduced rate that applies to residential building and extension work. This is applied to the full subtotal including fees. Professional fees from architects, engineers and surveyors are subject to the standard rate of 23%. Confirm with your architect whether their quote is inclusive or exclusive of VAT.

On a €100,000 pre-VAT extension subtotal, VAT adds approximately €13,500.

6. Contingency

A 10–15% contingency on construction cost is essential on an extension. Two reasons:

1. You're connecting to an existing structure. Until walls are opened up, nobody knows exactly what's behind them — old wiring, single-leaf brick, no cavity, a wall that isn't where the drawings said it was.
2. Material prices move. Most builder quotes hold for 30–90 days. If your build runs 4–6 months, the back half of the project is exposed to whatever has happened to timber, steel, or concrete prices in the meantime.

On an €84,000 build: €8,400 minimum, ideally €12,600.

Full Example: 30m² Mid-Range Rear Extension, Dublin

Total realistic budget including contingency: €128,517.

A 40m² version of the same extension comes in at roughly €150,000 – €175,000 all-in, and an 80m² two-storey extension (40m² footprint over two floors) at mid-range spec can hit €290,000 – €340,000. The rate per square metre stays in the mid-range band — total cost scales with size and any spec upgrades.

Types of House Extension — When Each Makes Sense

Per-m² extension rates don't really vary by type — the same trades and materials go into each. The differences below are about planning, layout, and value-for-space, not different per-m² costs.

Single-Storey Rear Extension

The most common extension type in Ireland. You're extending the back of the house, usually to open up the kitchen, create a kitchen-living-dining space, or add a utility and second living room. A single-storey extension under 40m² to the rear is usually exempt from planning, which keeps fees and timelines down. The right call when you need more ground-floor space and the back garden has the depth to give.

Single-Storey Side Extension

Builds out into the side passage or driveway space. The roof structure is often simpler than a rear extension, but side extensions always require planning permission in Ireland — there is no exemption for side extensions to the side or front of a house. Budget the same planning fee stack as any other planned extension.

Two-Storey Extension

Best value for total floor space added — you're already paying for foundations and a roof, so adding a second storey spreads those fixed costs across twice the floor area. Always requires planning permission. Typically takes 4–6 weeks longer to build than a single-storey of equivalent footprint because of the additional structural work and scaffolding. The right call when you need both extra ground-floor space *and* an extra bedroom or bathroom.

Wrap-Around Extension

Combines a rear and a side extension into a single L-shaped build. The most disruptive option on a constrained site, but the biggest space gain — typical wrap-arounds add 40–60m² and completely re-shape the ground floor of a semi-detached house. Best for older terraces and semis where the existing layout is the bottleneck.

Kitchen Extension

Not a separate technical category — a "kitchen extension" is typically a single-storey rear extension where the kitchen will move into the new space. Worth budgeting separately because the kitchen itself (units, worktop, appliances) is usually quoted on top of the extension build cost. Budget €10,000 – €25,000 for the kitchen alone at mid-range, or €30,000+ for a high-end bespoke kitchen with stone worktops and integrated appliances.

How Much Does Location Affect the Cost?

Significantly. Dublin is the most expensive county to extend in, and all other counties are priced relative to Dublin rates. Regional cost variation works out at:

That means the same 30m² mid-range rear extension that comes in at around €118,000 in Dublin would come in at roughly €108,000 in Cork, €104,000 in Galway, or €101,000 in Donegal — a real saving of €10,000 – €17,000 on the all-in cost, before contingency. Most counties cluster in the 5–15% cheaper band; only the western and northwestern counties hit the 18% end.

For a county-specific number, use the build cost calculator at BeforeYouBuild.ie.

Extension vs Renovation vs New Build — Which Makes Sense?

This is the comparison that actually matters when you're weighing options. Here's how the per-m² mid-range rates stack up in 2026:

Extension is the most expensive type of work per square metre — in most cases more than a new build. The reason: extensions involve all the same trades as a new build but on a smaller, more constrained site, with the added complexity of tying into an existing structure and protecting what's already there.

The question is rarely "which is cheapest per m²?" — it's "which gets me what I actually need?" If you need more floor space and the existing house is fundamentally sound, an extension is almost always the right answer. If the existing house is tired but the layout works, renovation is cheaper. If neither the layout nor the house is salvageable, a new build often makes more financial sense than a major extension plus full renovation.

Three Things That Catch Extenders Out

1. The exemption that wasn't. A single-storey rear extension under 40m² is exempt from planning permission only if it meets every condition: doesn't exceed the existing eaves height, doesn't reduce the open space behind the house below 25m², isn't on a protected structure, isn't in an Architectural Conservation Area, and is to the rear (not side or front). One condition fails and you need full planning permission. Get a Section 5 declaration from your local authority in writing before you start work if there's any doubt — it costs €80 and saves a lot of grief later.

2. The drainage discovery. Most older Irish houses have foul and surface water drains running across the back garden, often unmarked on any drawing. If your extension lands on top of one, the drain has to be diverted before work starts. Budget €500 – €2,500 for a single drain divert, more if it requires a new manhole.

3. The "match the existing" problem. Matching brick, render, slates, or fascia colour to a 30-year-old house is much harder than it sounds. Sometimes the original product is no longer made and the closest equivalent looks subtly wrong for years. Allow extra in your finishes budget for the joining strip between old and new, and ask your architect to plan for a deliberate visual change at the join rather than a doomed attempt to hide it.

Frequently Asked Questions

How much does a 30m² extension cost in Ireland in 2026?

A typical 30m² single-storey rear extension at mid-range spec costs €115,000 – €130,000 all-in in Dublin, including VAT, professional fees, and services. Outside Dublin, the same extension comes in 5–15% cheaper depending on county.

How much does a 40m² extension cost in Ireland in 2026?

A 40m² single-storey rear extension at mid-range spec costs €150,000 – €175,000 all-in in Dublin. At 40m² you're at the threshold for planning exemption, so confirm with your local authority before assuming planning isn't needed.

How much does a two-storey extension cost in Ireland in 2026?

A two-storey extension with a 40m² footprint (80m² total floor area) at mid-range spec costs €290,000 – €340,000 all-in in Dublin. Two-storey extensions always require planning permission.

Do I need planning permission for a house extension in Ireland?

A single-storey rear extension under 40m² is usually exempt, provided it meets every planning exemption condition. Side extensions, front extensions, two-storey work, anything over 40m² to the rear, anything affecting a protected structure, and anything in an Architectural Conservation Area all require planning permission.

How long does a house extension take to build in Ireland?

A typical single-storey extension takes 12–20 weeks from breaking ground to completion. A two-storey extension takes 18–28 weeks. Add 8–12 weeks for planning permission if required, plus 4–6 weeks for tendering and contracts.

Are there grants for house extensions in Ireland in 2026?

There's no direct grant for the cost of building an extension. However, if your extension upgrades the energy rating of part of the home, you may be eligible for SEAI grants on the energy-efficient elements — up to €12,500 for a heat pump system, up to €8,000 for wall insulation, and up to €5,600 for new windows and doors (new from March 2026).

What VAT rate applies to a house extension in Ireland?

Construction services for residential extensions attract VAT at the reduced rate of 13.5%. Professional fees from architects, engineers and surveyors are subject to the standard rate of 23%. Confirm with your architect whether their quote is inclusive or exclusive of VAT.

Is a single-storey or two-storey extension better value per m²?

A two-storey extension is better value per square metre of floor space added, because the foundations and roof costs are spread across twice the floor area. However a two-storey extension always requires planning permission and takes longer to build, so the right answer depends on how much extra space you need and how much disruption you can tolerate.

Get Your Own Estimate in 60 Seconds

Every extension is different. The figures in this guide are national averages for 2026 — your actual cost will depend on your county, your existing house, your spec level, and dozens of other factors.

Use the Before You Build build cost calculator to get a free, personalised estimate based on your specific project:

Get My Free Estimate → BeforeYouBuild.ie

It takes 60 seconds, requires no drawings, and gives you a realistic cost range you can take to your contractor, architect, or mortgage broker — before you commit to anything.

Key Takeaways

• A house extension in Ireland in 2026 costs €2,100 – €3,900 per square metre depending on quality and location
• A typical 30m² mid-range rear extension costs €115,000 – €130,000 all-in in Dublin including VAT, fees, and services
• Two-storey extensions are the best value per square metre for total floor space added, but always require planning permission
• Location matters — the same extension in Galway or Cork costs €10,000 – €17,000 less than Dublin
• Planning exemption for rear extensions under 40m² is conditional — confirm with a Section 5 declaration if in doubt
• Plan for 12% contingency, not 10% — extensions reveal what's behind existing walls
• There's no direct extension grant, but SEAI grants for heat pumps, insulation, windows and doors can apply to upgraded energy elements

*Data sources: SCSI House Rebuilding Guide (2024/2025), Buildcost Construction Cost Guide (H2 2025), Citizens Information (planning exemptions and Section 5 declarations), Revenue Commissioners (VAT), SEAI grant rates (March 2026). Figures reflect Q1 2026 rates.*