Extension cost calculator

The estimate starts from a cost per square metre for the type of extension you choose. A wrap-around and a single-storey side return are different jobs, and a single average across both would be useless for either. That rate is multiplied by your floor area, then adjusted twice: once for the finish level you pick, and once for the cost of labour where you live.

On top of the shell, the calculator adds only the extras that actually apply to your job. A kitchen fit-out, a bathroom, bifold doors and structural steel are each added when you say you need them, and left out when you do not. The statutory costs of building, meaning the planning application, building regulations, a structural engineer, and a party wall agreement where your property is terraced or semi-detached, are added on the same basis.

A contingency is then shown as its own line rather than folded into the total. That is deliberate: on a job of this size something unexpected is normal, and hiding it inside the headline figure would make the total look tidier and hide something real.

The result is always a range, never a single figure. A precise number would be a false promise, because two builders quoting the same job honestly can land a long way apart. Everything is rounded rather than shown to the pound, for the same reason.

Every figure behind this comes from a table we maintain by hand and review on a schedule. The date of the last review is shown on this page, so you can judge how current it is rather than take our word for it.

The figure covers the build itself plus the costs that come with it:

• The shell: foundations, structure, roof, and making it weathertight and habitable to the finish level you chose
• A kitchen fit-out, where you have said the extension includes one
• A bathroom or shower room, where you have said the extension includes one
• Bifold or sliding doors, priced per opening
• Structural steel, where the opening into the existing house needs it
• The planning application, building regulations, and a structural engineer
• A party wall agreement, on terraced and semi-detached properties
• A contingency, shown separately so you can see it

Where the calculator can also estimate what the work adds to your property, it shows that alongside the cost, including when the answer is that it does not pay back.

Anything not in the list above is not in the number. In practice the ones that catch people out are:

• Redecorating or altering the rooms the extension opens into
• Landscaping, and putting the garden back after the site is cleared
• Moving drains, or diverting a sewer running under the footprint
• Asbestos survey and removal in older properties
• Anything found once the ground is open, such as poor foundations, made ground, a high water table
• Somewhere to live if the house becomes unliveable during the work
• Finance costs, if you are borrowing to build

VAT is worth asking about specifically. Treatment varies with the property and the contractor, and a quote that omits it is not comparable with one that includes it.

Labour is the part of an extension that moves most by region, so it is the part we adjust. Every area carries a labour multiplier, and the calculator applies it to the labour element rather than to the whole figure, because materials cost broadly the same everywhere, and scaling them by region would overstate the difference.

Where we hold published data for your area, that area's own multiplier is used. Where we do not, the calculator falls back to the figure for your region and the page says so. It will never quietly borrow a nearby city's numbers and present them as local. A London figure on a Cumbrian postcode is worse than no figure at all.

If the number is roughly what you expected and you want to go ahead, the order of what happens next matters, and it does not start with a builder.

1. Check with your local council whether you need planning permission at all. Many extensions fall under permitted development, but the limits are specific.
2. Get a technical drawing. An architect is the best route. They visit, you explain what you want and where you want it, and you talk through what is actually possible before anything is drawn. You need a drawing to satisfy building regulations regardless, so this is not a step to skip.
3. Bring in a structural engineer if you are removing a load-bearing wall, which most extensions involve. They size the steel, and that size affects the price.
4. Then get quotes, at least three, from builders all working to the same drawing. Comparing quotes before you have a drawing compares guesses.

Only at that last step does the figure above do its real work: it tells you whether the quotes in front of you are roughly where they should be.

Choosing between builders is not only about the total. Ask to see previous jobs of the same type and read their reviews. Be clear about payment, and insist on paying stage by stage against work actually completed. Settle materials, timeline and details at the start rather than during, because that is where most surprises come from.

If a quote comes in below our range, that usually means a newer business building a reputation, or work that cuts corners. Above it usually means limited local supply, or a firm with higher overheads. Both can be the right choice. The question worth asking is what is included, not why the number differs.

Worth asking every firm you speak to:

• Is this a fixed price or an estimate, and what would change it?
• Does it include VAT, and are the statutory fees inside or outside the figure?
• What is the contingency, and who decides when it is spent?
• Who is applying for building regulations approval, you or me?
• What happens to the price if the foundations need to go deeper than assumed?