How Much Does a Dublin Extension Actually Cost?

So folks, the question I get asked more than any other — more than contracts, more than builders, more than planning — is this one.

How much is my extension going to cost?

And I'll be honest with you. It is the question I most want to give a straight answer to, and the one where a straight answer is hardest to give. Because the honest answer is: it depends. And I know that is the answer nobody wants to hear. So let me do better than that.

Let me give you the ranges. Real ranges. From real recent jobs in Dublin. Not the number on the back of an envelope your architect gave you at the first meeting, and not the number you saw on a renovation programme on RTÉ. The actual number. The eye-watering one.

First — what are we actually talking about?

A Dublin extension. Single-storey, to the rear of a semi-d or terraced house. That is the most common job I see. Family in Rathfarnham, family in Clontarf, family in Glasnevin — same house type, same ambition, different budgets.

A 30 to 40 square metre extension. Open plan kitchen and living area. Pitched or flat roof. Standard Dublin build. That is the job.

Now. The reality is this: in 2026, you are looking at build costs of between €2,200 and €3,500 per square metre, depending on specification. That is the builder's number, before anything else.

On a 40sqm extension, that is €88,000 to €140,000. Just the build.

And that is before the architect. Before the structural engineer. Before planning, if you need it. Before the QS, before VAT at 13.5%, before the skip, before the contingency. Before the kitchen you want to put in the new space.

What the total number actually looks like

I'll give you an example. A family in South Dublin came to me not long ago. They had a budget in their heads of €60,000. They had seen a show on RTÉ. Their architect had mentioned €1,500 per square metre at the initial meeting.

Their extension was 42 square metres. Here is what it actually cost.

Build cost: €118,000. Architect and structural engineer fees: €14,000. QS fee: €4,500. Planning application: €900. VAT at 13.5%: approximately €16,000. Contingency used: €8,000. Kitchen and fitted units: €22,000.

Total: €183,000.

Now. Was that a bad project? No. The build was excellent. The builder was fair, the finish was good, and the family are delighted with the space. But the number was a serious number. And they needed to know that before they started, not after they had fallen in love with the drawings.

That is the job I do.

Why the cost gap exists — and why it is not the architect's fault

Here is the thing. When your architect gives you a number at the early stages, they are giving you a ballpark. They are not lying to you. They are not being reckless. They are giving you the best estimate they have before the spec is written, before the builder prices the job, before they know how deep your foundations need to go or what they are going to find when they open the floor.

The gap between the architect's early estimate and the builder's tender price is normal. It is construction. It happens on residential projects right across Dublin and Ireland. What causes the gap to widen? Five things, in my experience.

One — specification creep. The job you described at the first meeting is not the job in the drawings six months later. Bifold doors instead of a French door. Underfloor heating added. Roof lights. Every decision is reasonable. The cumulative effect is a very different number.

Two — ground conditions. In Dublin, you do not always know what is under the floor until you are in it. Old drains, made-up ground, soft spots — these add cost. Contingency exists for this reason.

Three — services relocation. Moving the boiler, rerouting electrics, upgrading the fuse board — these are the hidden costs nobody puts in the headline number.

Four — finishes. And here is the reality. Finishes are 40% of the eye-watering bit on most jobs. Tiling, flooring, kitchen, sanitaryware, ironmongery, paint — this is where the money goes on a Dublin extension.

Five — VAT. Thirteen and a half percent. A lot of people forget this until the invoice arrives.

What actually keeps the cost down

Less is more. That is my philosophy on almost everything, and it applies here too.

Lock the spec before you go to tender. Every change after the contract is signed is a variation. And variations are the most expensive way to build anything — on any project, in any part of Ireland.

Choose your builder carefully. The cheapest tender is almost never the cheapest build. I've seen it happen too many times on Dublin jobs — a builder who prices low, then needs variations to survive, ends up costing you more than a builder who priced fairly and delivered.

Get a cost plan done before you go to tender. Not after. Before. It will tell you where you are before you commit, and it gives you something real to measure the builder's price against when the tenders come back.

And — I will say this gently — be honest about the budget. The worst thing that happens on a residential project anywhere in Ireland is when the homeowner has a real budget and a different number in their head. It is my job to find the real number. And the sooner we find it, the better for everyone.

What a cost plan actually covers

A cost plan is not a builder's tender. It is an independent estimate of what the job should cost, prepared before the builder comes anywhere near it.

It covers the build, the finishes, the professional fees, the contingency, and the VAT. It tells you whether what you want to build is within what you can afford to spend. And if it is not, it tells you that before you have spent money on detailed drawings that cannot be built.

Most projects that go wrong do not go wrong over bricks and mortar. They go wrong because expectations were not managed. That is the whole thing. And a good cost plan, done early, is the single biggest thing I know that manages them.

A rough guide to Dublin extension costs in 2026

These are ranges only. Your job will depend on your spec, your site, and your builder. But they are real ranges, from real recent work.

Basic finish, single-storey rear extension, 30–40sqm, Dublin: €95,000 to €130,000 all-in including fees and VAT.

Mid-range finish, same size: €130,000 to €175,000 all-in.

High specification, roof lights, underfloor heating, high-end kitchen: €175,000 to €230,000 and above.

Two-storey extension adds roughly 60–70% to the single-storey cost for the same footprint.

Anyway, that is my way of saying: the number is a serious number. It always was. But if you know the real number upfront, you can plan for it. And that is a much better place to be standing than discovering it halfway through the build.

Download the Dublin Extension Cost Reality Check

I have put together a one-page PDF — real ranges from real recent Dublin jobs, broken down by build type, finish level, and size. No hype, no rounding down, no television numbers.

Download it below. And if you want a proper cost plan for your specific project, here is how that works.

Download the Dublin Extension Cost Reality Check → [LEAD MAGNET LINK]

If you want a proper cost plan, here's how that works → roryconnollyqs.ie/cost-plan

If I can help in any way, let me know.