Planning in Wales

Wales has its own planning system, and it is worth starting there, because much of what you will read online describes England.

Here, a single-storey rear extension is permitted development if it projects no more than three metres beyond the original rear wall of a terraced or semi-detached house, or four metres for a detached one. It also has to stay under four metres in height and sit no higher than the existing eaves. No more than half the land around the original house can be built on.

What Wales does not have is England's Larger Home Extension Scheme. The six and eight metre limits quoted on most extension websites are English, and the prior approval process that goes with them does not exist here. If you want to go further than three or four metres in Wales, you apply for planning permission.

A few other Welsh particulars are worth knowing. A two-storey rear extension can project four metres at ground floor but only three at first floor, and must leave at least ten and a half metres to the rear boundary. Two-storey rear extensions are not permitted development at all in a conservation area. Cardiff has a great many of those, including much of Roath, Cathays, Pontcanna and Llandaff, with more across Penarth. The "original house" means the house as it stood in 1948, or as first built, so a previous owner's extension counts against your allowance even though you did not build it.

If your house is listed, none of this applies. You will need listed building consent, which since November 2024 has operated under the Historic Environment (Wales) Act 2023. We have written more about that in a recent journal article, here.

Where a project is permitted development, we usually suggest applying for a Certificate of Lawfulness. It isn't compulsory, but it is a good deal cheaper than discovering a problem when you come to sell.

What it costs

In Cardiff and the Vale in 2026, a single-storey extension built to a good standard costs roughly £2,500 to £3,800 per square metre, before VAT and fit-out. A twenty square metre rear extension therefore sits somewhere around £50,000 to £76,000 to build.

That is the building alone. Structural engineering, planning and building regulations fees, and party wall costs where a neighbour is affected all sit on top. So do kitchens, joinery and landscaping, and those last few are usually what moves a budget. We would always allow a contingency of around ten per cent.

Specification moves the numbers more than size does. Two extensions with the same footprint can differ by half again depending on the glazing, the roof, and how the new part meets the old. Part of our job in the first few weeks is to tell you honestly which version of your idea your budget will buy.

How long it takes

From first meeting to starting on site, six to twelve months is normal. Survey and design take four to eight weeks. A householder planning application in Cardiff or the Vale takes eight to twelve weeks from validation, sometimes longer. The technical drawings and tender package take another six to eight weeks after that. Building a single-storey rear extension usually takes ten to sixteen weeks, and two storeys sixteen to twenty-four.

Most people underestimate the first half and overestimate the second. The design and consent stage is where the time goes, and it is also where the decisions that matter are made.

How we work

We begin with a measured survey and a feasibility study that tests two or three options against your budget before anyone commits to one. From there we develop a single design and apply for planning or a Certificate of Lawfulness. Then we prepare the technical package, get tenders from contractors we know, and look after the contract on site.