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

If you like where you live but the house has run out of room, an extension is the obvious answer. The second question always follows immediately: does it pay for itself?

There is a lot of confident nonsense written about this, most of it a percentage with no source attached. So here is what is actually published, who published it, and where it stops applying to your house.

What the published research actually says

The most useful independent figures come from Nationwide, because they are derived from the society’s own mortgage lending data rather than from a survey of intentions. Their 2023 analysis, covering the twelve months to June 2023, breaks the uplift down by what you add rather than by how big it is:

What Nationwide's research attributes to each home improvement Nationwide's 2023 analysis of its mortgage lending data attributes a 5 per cent price increase to a 10 per cent increase in floor space on a typical house, 6 per cent to an additional bathroom on an average house, 14 per cent to an additional double bedroom on an existing two bedroom house, and up to 25 per cent to a loft conversion or extension adding both a double bedroom and a bathroom to a three bedroom, one bathroom house. These are national averages, not a valuation of any individual property. What adds value, by what you add — Nationwide, 2023 0% 10% 20% 30% 10% more floor space typical house +5% An extra bathroom average house +6% An extra double bedroom on an existing two-bed +14% Bedroom and bathroom together on a three-bed, one-bath house up to +25% National averages from lending data — not a valuation of your house. The ceiling price on your street still applies, and it is local. Ask an agent, not a builder. Source: Nationwide, “Bedrooms, bathrooms and extensions: what adds value to your home in 2023?”, 12 months to June 2023.
The pattern is consistent across the research: it is the bedroom count and the bathroom count that move a valuation, not floor area on its own. An extension that adds a lot of square metres without changing either number is the one most likely to disappoint on paper.

Where that stops applying to your house

Two caveats do most of the work, and neither is in the headline percentage.

The first is the ceiling price of your street. Every road has a number above which buyers stop competing, and it is set by the road rather than by your house. If you are already near it, an extension buys you comfort rather than equity. That is a perfectly good reason to build — it is just not a financial one, and you should know which one you are doing.

The second is what you lose. Filling a side return with a bigger kitchen changes no room count. Converting the fourth bedroom into an en-suite adds a bathroom and removes a bedroom. On Nationwide’s own numbers that trade can go either way, and it depends entirely on what the house had to begin with.

Grey fitted kitchen units — All Aspects Construction, Cardiff Tiled walk-in shower enclosure — All Aspects Construction, Cardiff
Two finished rooms from our own work. On the published research the shower room on the right is the one with a number attached to it — a bathroom is a countable thing, and a nicer kitchen is not.

What we will and will not tell you

We will price the build, itemised, with the structure separated from the specification so you can see which parts are fixed and which are choices. We will tell you honestly whether what you are describing changes the bedroom or bathroom count, because that is the part the research says matters.

What we will not do is put a figure on what your house will be worth afterwards. We are builders. A local estate agent who sells on your street every month will give you a far better answer to that question, and it costs nothing to ask them before you commit.

Worth doing first

Before you commission a single drawing, ask two agents what your street’s ceiling is and what a four-bed on it actually achieves. It takes a phone call, and it is the cheapest piece of research in the whole project.

Quality still matters

A poorly executed extension can take value off a house rather than adding it. A new addition has to sit properly against the original — the brick, the render line, the roof pitch, the way the join is detailed. That is not a premium upsell, it is the difference between an extension that reads as part of the house and one that reads as something bolted on.

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