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

If you own a Victorian terrace in Roath, a bay-fronted house in Pontcanna or a 1930s semi in Cyncoed, you have probably wondered what an extension would actually cost. The honest answer: it depends — but not as much as the internet would have you believe.

As rough guidance only, a well-built single-storey extension in Cardiff typically falls between £1,800 and £2,500 per square metre in 2026, with double-storey work often costing less per metre because the foundations and roof are shared. You can get a tailored indicative range in about a minute with our cost calculator — then treat it as a starting point for a conversation, not a quotation.

What moves the price in Cardiff specifically: the city's Victorian and Edwardian terraces almost always involve party wall agreements on both sides, and many have no side access, which affects logistics. Properties in conservation areas such as Pontcanna, Llandaff and Roath Park need full planning applications and designs that satisfy the council's conservation officers. And opening up the rear of a period terrace properly usually means structural steelwork — worth it, but it has to be engineered, not guessed.

The costs people forget: design and structural engineering fees, planning or lawful development certificates, Building Regulations fees, party wall surveyors, and VAT. A proper proposal should make all of this visible before you commit — ours always do.

Be wary of any builder who prices your Cardiff extension over the phone or from a photograph. A reliable figure comes from a site visit, a defined scope and an itemised proposal. That is exactly how we work — the initial consultation and site visit cost you nothing.

We deliver extensions across the whole city — see our builders in Cardiff page for how we work locally, or request a consultation and we will give you a straight answer about what your project is likely to involve.