My extension quote seems expensive: how do I know?

An extension is the largest single quote most homeowners will ever read, and the hardest to judge, because there is no everyday reference point for what a wall, a roof, and a slab should cost. A figure that looks shocking can be completely fair, and a figure that looks reasonable can be missing half the job. The way to tell them apart is to anchor on the per-square-metre build cost and then check what the quote actually includes.

This guide shows how. For the underlying ranges, start with the house extension cost guide, and for the general method, how to tell if a quote is too high.

Start with the per-square-metre anchor#

The most useful sense check is build cost per square metre of floor area. A single-storey extension typically runs £1,800–£3,000+ per m² in 2026, with London and the South East at the top and beyond. Multiply by your floor area:

• A 15m² extension at £2,200/m² is roughly £33,000
• A 25m² extension at £2,600/m² is roughly £65,000

If your quote lands near the per-m² range for your area, it is in the right ballpark. If it sits well above with no clear reason, that is your signal to dig into the detail.

Check which stage the quote covers#

A per-m² figure usually describes the shell and basic finish. The number climbs once you add the things that make an extension usable, and a major source of "this seems expensive" is simply that one quote includes them and another does not:

• Kitchen or bathroom fit-out
• Flooring
• Decoration
• Moving or connecting services (drainage, gas, water, electrics)
• Making good the garden after groundworks

Before you judge the total, establish whether you are looking at a shell price or an all-in price. They are different jobs.

Where an extension quote legitimately rises#

Rule these in before deciding a quote is too high:

• Groundwork unknowns. Sloping sites, poor soil, deep foundations near trees or drains. These are real and sometimes unavoidable.
• Structural complexity. Large openings needing steel, knocking through to the existing house, or going two storeys.
• High glazing content. Bifold doors, roof lanterns, and large windows are expensive and push the price up legitimately.
• Restricted access. A site machines and materials cannot easily reach costs more in labour and time.
• Party Wall and planning costs. Surveyor fees and planning conditions add real money. See the hidden costs guide.

Where extension quotes get padded#

The same signals as any inflated quote, amplified by the size of the job: a lump sum with no breakdown, provisional sums set high to keep the headline down, labour that does not reconcile to a believable programme, and a margin well above the normal 15–25% with nothing premium to justify it. On a £50,000 job, a few padded lines add up fast, which is exactly why an itemised quote matters most here.

Before you commit#

1. Does the total land near the per-m² range for your area and floor size?
2. Is it a shell price or an all-in price, and which do you need?
3. Are groundworks and foundations itemised, with a realistic contingency?
4. Are provisional sums (kitchen, bathroom, glazing) named with figures?
5. Is the deposit 10–15%, with staged payments against milestones?

On a project this size, an independent check pays for itself many times over. Paste or upload your extension quote into Check the Quote and we benchmark every line against current UK rates for your postcode, flag what is above the fair range, and tell you what the quote has left out. Your first check is free.