How Much Does a Single Storey Extension Cost in the UK?

Realistic price ranges in 2026

As a rough guide, expect to pay somewhere between £1,800 and £3,000 per square metre for a finished single storey extension. A modest 15m2 rear extension might land around £30,000 to £45,000, while a larger 25m2 to 30m2 open plan kitchen and dining space often runs from £50,000 to £75,000 or more.

Costs in and around Scunthorpe tend to sit slightly below central London and the South East, which works in your favour. That said, the finish you choose makes a big difference, so two extensions of the same size can vary by tens of thousands of pounds.

• Basic finish: £1,800 to £2,200 per m2
• Mid range with good kitchen and glazing: £2,200 to £2,600 per m2
• High specification with bifolds and underfloor heating: £2,600 to £3,000+ per m2

What pushes the price up or down

The biggest variables are groundworks and the spec inside. A simple flat footprint on stable ground is far cheaper than a site needing deep foundations, drain diversions or tree root considerations, all of which are common across older North Lincolnshire properties.

Glazing is the other major factor. Large bifold or sliding doors, roof lanterns and structural openings between old and new rooms all add cost but transform how the space feels and performs.

Costs people forget to budget for

The build itself is only part of the picture. Set aside a contingency of around 10 to 15 percent for the unexpected, such as poor ground conditions or upgrading old wiring and heating to cope with the new space.

Professional fees and statutory costs also add up before a single brick is laid, so factor these in early rather than treating them as an afterthought.

• Architectural drawings and structural engineer calculations
• Building Control fees through North Lincolnshire Council or an approved inspector
• Planning permission, if your project falls outside permitted development
• Party wall agreements where you share a boundary

Getting an accurate quote

Online calculators are a starting point, but only a site visit gives a figure you can rely on. A good builder will want to see your ground, access and existing services before committing to a price.

Always compare like for like. A cheaper quote often excludes items such as the kitchen, decoration or VAT, so ask exactly what is and is not included before you decide.