How much does a house extension cost in South Wales and what affects the price?

A house extension can give you the space you need without the upheaval of moving. But the first question most homeowners ask is a sensible one: how much will it cost?

There is no honest fixed answer without looking at the property and understanding the job. Two extensions with the same floor area can have very different prices. Here is how to approach your budget and what tends to move the figure up or down.

What should you budget for an extension?

As a broad starting point, a straightforward single-storey extension in South Wales might typically be budgeted at around £2,000–£3,000 per square metre for the building work, with more complicated or higher-specification projects costing more.

On that basis, a 20-square-metre extension could mean roughly £40,000–£60,000 for the building work. Treat this as an early planning guide, not a quote or an all-in project total.

Check whether any figure includes VAT, professional fees, kitchen fittings, flooring, decorating and outside work. A square-metre estimate is only useful when you know what sits behind it.

Size, shape and number of storeys

More space generally means more materials and labour, but cost does not rise in a perfectly straight line. Even a small extension needs foundations, a roof and connections to the existing house.

A simple rectangular layout will typically be easier to build than one with several corners, unusual roof details or large areas of glazing.

A two-storey extension may offer better value per square metre because some groundwork and roofing costs serve both floors. However, the overall bill will usually be higher, and structural work can be more involved.

Ground conditions and site access

What is below ground can have a big effect on the budget. Foundation design, sloping land, drainage routes and nearby trees can all influence the work needed.

Access matters too. On a tight terraced-house plot, getting machinery and materials to the rear may take more time and handling than on a property with a wide driveway.

Before comparing prices, make sure each builder has considered:

• Access for deliveries, machinery and waste removal
• Ground levels and likely foundation work
• Existing drains and any proposed alterations
• Protection for the house, garden and neighbouring boundaries

Some conditions cannot be confirmed until work starts, so ask how unexpected work would be priced and agreed.

Structural work and connecting old to new

An extension is not just a new room attached to a wall. Opening up the existing house may involve structural steelwork, temporary support and changes to the layout.

Moving a boiler, upgrading heating provision or altering electrics can also add to the cost. The condition of the existing building matters, particularly where new work meets older walls, floors and roofs.

These details are worth investigating early rather than leaving them as assumptions.

Kitchens, bathrooms and finishes

The finished specification can make a substantial difference. A basic utility room and an open-plan kitchen extension may have similar footprints but very different budgets.

Kitchen units, appliances, worktops, bathroom fittings, glazing and floor finishes all affect the total. Large sliding doors and rooflights can also change both material and installation costs.

Choose your priorities early. Spending on the features you will use every day often makes more sense than stretching the budget everywhere.

Allow for costs beyond the build

Your overall budget may also need to cover drawings, structural design, surveys and application or inspection charges. Check with your local authority about the permissions and approvals relevant to your plans.

Keep a contingency too. Around 10–15% is a common budgeting allowance, although projects with greater uncertainty may need more.

Get a quote you can understand

At P&S Construction, our advice is simple: agree the scope before comparing the price. Ask for clear inclusions, exclusions and allowances, and understand how changes will be handled.

A realistic extension budget starts with your property, your plans and a proper conversation—not just a headline price per square metre.