Rear house extension with brickwork and render — All Aspects Construction, Cardiff

Every extension conversation starts in the same place, and it is not the design. It is what is this going to cost me? — and the honest answer is that anyone who gives you a firm figure before seeing the house is guessing.

That is not a dodge. It is the single most useful thing we can tell you, because the range of legitimate answers is enormous and almost all of the variation comes from things you cannot see in a photograph. What we can do is show you the published research, be straight about its limits, and then explain exactly which local factors push a Penarth, Barry or Cardiff job up or down within it.

On this page The published figures · Why there is no Welsh figure · What moves the number here · The VAT rule

The published figures, and who published them

The most widely cited independent source is the HomeOwners Alliance, which publishes cost guidance for house extensions across the UK. Here are their 2026 rates per square metre, drawn to scale:

Published extension cost ranges per square metre The HomeOwners Alliance's 2026 guidance puts a single-storey extension at 1,800 to 3,000 pounds per square metre, a two-storey extension at around 1,900 pounds per square metre, and a single-storey extension in London at 2,300 to 3,400 pounds per square metre. A two-storey extension is therefore cheaper per square metre than a single-storey one. No Wales-specific figure is published; the guidance states only that costs in Wales sit below London and the south east. Published cost per m² — HomeOwners Alliance, 2026 £0 £1,000 £2,000 £3,000 Single-storey UK guidance £1,800 £3,000 Two-storey UK guidance around £1,900 — cheaper per m² than going out Single-storey London £2,300 £3,400 Wales: no published figure at all The guidance says only that Wales sits below London and the south east. It does not say by how much. Source: HomeOwners Alliance, “House Extension Costs”. Reproduced with attribution — these are not All Aspects prices.
The bars are the published ranges drawn to a common scale, so the overlap is visible: the top of the ordinary UK single-storey range already reaches into London territory. Specification, not geography, is doing most of that work.

For whole jobs rather than rates, the same guidance gives:

Single-storey extension size Published cost
20m²£40,000 – £56,000
30m²£60,000 – £84,000
50m²£100,000 – £140,000

Those are the HomeOwners Alliance’s numbers, not ours. We are reproducing them because they are the clearest independent starting point available, and because the two-storey line surprises people every time: going up as well as out is usually cheaper per square metre than going out alone, since you are already paying for the foundations, the roof and the site setup.

Why there is no Welsh figure — and be suspicious of anyone who quotes one

Here is the part that most cost guides skate over. The HomeOwners Alliance notes that extension costs in London and the south east run higher than in the north, the midlands, and in Wales and Scotland — but it does not publish a Wales figure. Nor does anyone else, reliably.

So when a website confidently tells you an extension in Cardiff costs a specific amount per square metre, that number has almost certainly been derived by applying a guessed regional discount to an English average. It is not research. We would rather tell you the national range is your ballpark, tell you Wales generally sits at the lower end of it, and then quote your actual house.

What actually moves the number around here

In our experience across Penarth, Barry, Cardiff, Caerphilly and Pontypridd, the same five things account for most of the spread between two superficially identical extensions.

1. Whether you need planning permission at all

This is the biggest single lever, and it is a rules question rather than a building one. If your extension fits inside permitted development you save the application fee, the drawings that go with it, and — more importantly — the weeks. In Wales that means a single-storey rear extension of no more than 4m beyond the original rear wall and no more than 4m high.

Single-storey rear extension limits in Wales In Wales a single-storey rear extension may project up to 4 metres beyond the original rear wall and be up to 4 metres high, with no separate allowance for detached houses. England's Larger Home Extension prior-approval scheme, which permits 6 metres on an attached house and 8 metres on a detached one, does not apply in Wales. How far a single-storey rear extension can go — in Wales existing house original rear wall 4m +2m 4m max height England only The 6m / 8m “Larger Home Extension” prior-approval route does not exist in Wales. permitted development in Wales needs full planning permission here
Wales applies one 4m limit to every house type. There is no detached-house allowance and no prior-approval route to 6m or 8m — those are England’s rules. Source: gov.wales, “Planning permission: extensions”.

Note what is not on that diagram: the 6m and 8m figures that dominate UK search results. Those come from England’s Larger Home Extension prior approval scheme, and Wales has no equivalent. If a budget has been built around a 6m extension going through without full planning permission, that budget is wrong in both money and time.

2. Whether the house is in a conservation area

Central Penarth, the enlarged Pontypridd town centre boundary, parts of Caerphilly and a good deal of Cardiff sit inside conservation areas. Inside one, side extensions, two-storey rear extensions and any roof extension all lose permitted development rights. That does not make them impossible — it makes them an application, with the design and professional fees that come with it, and often material specifications that cost more than the standard.

3. Access

A dock-era terrace in Cadoxton with no rear access is a fundamentally different job from a post-war semi in Bedwas with a driveway to the back gate. If every barrow of muck out and every pack of blocks in has to travel through the house, that is labour, protection and time, on every single day of the build.

Block-paved side passage between two houses — All Aspects Construction, Cardiff
A side passage like this one, from one of our own jobs, is the whole access question in a photograph. Everything that goes into the build and everything that comes out of it travels through that gap.

4. Ground and levels

On the valley sides around Pontypridd and Caerphilly, the back of a house can sit a full storey below the front. Level changes drive retaining structures, longer drainage runs and awkward access, and all of those are decided at foundation stage. A flat-site price applied to a sloping plot is not a quote, it is an opening bid.

Bricklaying and brickwork in progress — All Aspects Construction, Cardiff
Brickwork on one of our sites. What the ground turns out to be is settled below this line, at foundation stage, and it is the one thing no quote can fully price from the pavement.

5. What you specify

The structure is the structure. Almost the entire spread between the bottom and the top of the published range is glazing, roof lights, floor finishes, heating and kitchen — two identical shells can differ enormously on what goes inside them. That is why we price the specification separately from the build, so you can see what you are actually choosing rather than being handed one number.

Grey fitted kitchen units — All Aspects Construction, Cardiff Tiled walk-in shower enclosure — All Aspects Construction, Cardiff
Two finished spaces from our own work. Nothing here is structure — it is all specification, and it is where the difference between the bottom and the top of the published range actually goes.

The VAT rule worth checking before you budget

Most building work is charged at the standard 20% VAT rate. There are two situations where it drops to 5%, and both are set out in HMRC’s VAT Notice 708:

When building work is charged at 5 per cent VAT instead of 20 Extending or enlarging an existing dwelling is always standard rated at 20 per cent VAT. The 5 per cent reduced rate applies where a property has not been lived in for at least two years before work starts, or where the work changes the number of dwellings in the building. 20% or 5%? The VAT question worth asking before you budget Extending or enlarging a house you live in the ordinary case 20% standard rated Not lived in for at least two years before you start VAT Notice 708 s.8 5% reduced rate The number of dwellings in the building changes VAT Notice 708 s.7 5% on the conversion work The catch people miss An extension or enlargement stays at 20% even on a property that has stood empty for years. The 5% rate covers renovating what is already there, not adding to it. Source: GOV.UK, Buildings and construction (VAT Notice 708), sections 7 and 8. Confirm eligibility with your accountant.
Worth checking before you set a budget: on an empty property the difference between 20% and 5% is real money. It never applies to the extension itself.

The catch, and it is the one people get wrong: the reduced rate never applies to an extension or enlargement, however long the property has stood empty. Section 8.1.1 of the notice is explicit about it. The 5% rate is for renovating what is already there, or for changing how many dwellings the building contains — a house into flats, or several units back into one.

If you have bought somewhere that has genuinely been empty for two years or more, that is worth raising with us and with your accountant at the quoting stage rather than after. On a full renovation the difference between 20% and 5% is real money, and the evidence (Electoral Roll, Council Tax records, or the council’s empty property officer) is easier to gather before work starts.

What we do instead of a price list

We visit, we measure, we look at the access and the ground, and we tell you what the rules allow on your specific address before anyone draws anything. The quote separates structure from specification so you can see which parts are fixed and which are choices. Consultations and estimates are free and carry no obligation.

If the honest answer is that your idea does not work within permitted development and a full application is the sensible route, we will say that at the first visit rather than three weeks in.

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