Builders in Cardiff
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Trusted Builders in Cardiff

We are a Penarth firm covering Cardiff, and we would rather say that plainly than pretend otherwise. The office is ten minutes from Grangetown and fifteen from the Bay, which is close enough to run a Cardiff site properly and honest enough that you know who you are dealing with.

The thing worth knowing when you cross the boundary is that Cardiff Council is a different local planning authority from the Vale of Glamorgan. Same national permitted development rules — they are set in Welsh law, not by councils — but different validation requirements, different officers, different local policy on top. A builder who tells you Cardiff works the same as Penarth has not applied in both.

Planning in Cardiff: the rules that are local, and the ones that are not

The permitted development limits themselves are Welsh, so they do not change at the city boundary: 4m for a single-storey rear extension, 4m in height, 3m eaves within 2m of a boundary, no more than half the surrounding land covered, and the flat rejection of England’s 6m and 8m prior approval route.

What is genuinely Cardiff-specific — and what catches out landlords and anyone converting — is the Article 4 direction covering Cathays, Roath and Plasnewydd. Inside that area, changing a normal family house (planning class C3) into a small house in multiple occupation (C4) needs planning permission, where elsewhere it would not.

The council then applies a concentration test. It draws a 50m radius around the property and counts the existing HMOs: above 20% in Cathays and Plasnewydd, or 10% elsewhere in the city, the concentration is treated as too high. That policy sits in the HMO supplementary planning guidance adopted in 2019. If your project is a conversion rather than an extension, it is the first thing to check, because it decides whether the scheme exists at all.

Cardiff also has a substantial number of conservation areas, and the same national rule applies inside every one of them: no roof extension or dormer under permitted development, no side extension, no rear extension above single storey. Worth confirming your street before the design stage, not after.

The other limit that does a lot of work in Cardiff is the loft volume allowance, because the city is unusually terrace-heavy. A terraced house gets under 40m³; a semi or detached gets under 50m³. Across Canton, Roath, Grangetown and Cathays that 10m³ difference is routinely what separates a rear dormer that needs no application from one that does — and on a narrow Victorian terrace the allowance is used up faster than people expect.

Two more that catch Cardiff terraces specifically: within 2m of a boundary the eaves of an extension cannot exceed 3m, which on a tight city plot is often the real height constraint rather than the 4m overall limit; and no more than half the land around the original house may be covered, which on a small rear yard is reached sooner than on a suburban plot.

The houses we work on in Cardiff

Cardiff’s housing is too varied for one useful generalisation, so it is worth splitting by what a project actually has to deal with:

• Victorian and Edwardian terraces through Canton, Pontcanna, Roath and Grangetown. Narrow frontages, long rear returns, and the classic side-return infill opportunity. These are also the houses where the 40m³ loft allowance and the 3m eaves rule bite hardest.
• The Article 4 belt — Cathays, Roath and Plasnewydd. Structurally the same terraces, but with a planning layer on top that decides whether a conversion is possible at all.
• Interwar and post-war stock out through Whitchurch, Rhiwbina, Llanishen and Heath. Wider plots, the larger 50m³ loft allowance, and the most straightforward extending in the city.
• Larger period houses around Penylan, Cyncoed and Llandaff, where the work is more often full renovation and reordering than adding floor area — and where a conservation area designation is more likely.

For standalone specialist trades — carpentry, plumbing or electrical work as a job in its own right rather than part of a build — our sister companies in the group cover Cardiff directly and will usually be the better call.

Areas We Cover in Cardiff

Cardiff Builders — Your Questions Answered