Cost guide · Updated [month 2026]
How much does an extension cost in Spalding?
Most single-storey extensions we build in South Holland cost between [£X and £Y], depending on the size, the roof and the finish. Here’s what drives the price, with typical ranges from our own recent jobs.
Typical prices from our recent jobs
| Job | Typical range | Usually includes |
|---|---|---|
| Single-storey rear extension around 3 × 4 metres | [£ range] | Footings to a plastered, ready-to-decorate room, including the knock-through |
| Conservatory replacement similar footprint | [£ range] | Old conservatory removed, new insulated room, opened into the house |
| Knock-through with a steel beam one load-bearing wall | [£ range] | Steel supplied and fitted, building control, plastering and making good |
| Garage conversion single garage | [£ range] | Door opening bricked up with a window, insulation, plastering |
| Porcelain patio per square metre | [£ range] | Dig-out, sub-base, porcelain laid and pointed |
Ranges are from our own jobs in [year], [including / excluding] VAT. They’re a guide, not a quote: your written quote is the price that counts.
What makes the price go up or down
Size and shape
A simple rectangle costs less per square metre than an L-shape or one with lots of corners. The first few square metres cost the most, because footings, a roof edge and a knock-through are needed whatever the size.
The roof
A flat roof is usually the most economical. A parapet wall, a roof lantern or a pitched roof that ties into the house all add cost, and they also change how the extension looks and how much light you get.
How much of the house is opened up
Taking out the old back wall needs steel beams sized by a structural engineer. A single beam over a doorway-sized opening is a small part of the cost; picture-frame steels across the full width of the house are a bigger one.
The ground
Much of South Holland is soft fenland, and building control sometimes ask for deeper foundations than usual. Drains running under the new footprint may need moving, or an agreement with Anglian Water to build over them. We look at both at the site visit and say in the quote how they’re handled.
The finish
Bifolds, roof lanterns, underfloor heating and the kitchen itself can be as much again as the building work. We’ll be clear about what our price covers and what you’re buying separately.
Costs that sit outside the build
- Drawings, and planning permission if you need it
- Structural engineer’s calculations
- Building control fees
- A party wall surveyor, if your neighbours want one
- Kitchen, flooring and decorating, unless they’re in the quote
What’s in our written quote
After a free site visit, you get a written quote that says what’s included, what isn’t, how the ground and drains are dealt with, and a realistic timescale. If you agree it at your home, you normally have 14 days to change your mind.
Related questions
Are your quotes free?
Yes. Ash will come and look at the job, talk it through with you and send a clear written quote: what’s included, what isn’t, and a realistic timescale. There’s no charge and no obligation. If you agree the work at your home, you normally have 14 days to change your mind under the Consumer Contracts Regulations.
Do I need planning permission for a single-storey extension?
Often not. Many single-storey rear extensions count as permitted development: up to 3 metres deep on a semi or terraced house, 4 metres on a detached house, and no more than 4 metres high. Bigger ones, up to 6 or 8 metres, can go through the council’s prior approval process. Listed buildings, conservation areas and some newer estates have different rules, so check with South Holland District Council first.
How long does a single-storey extension take?
Most single-storey extensions take [8 to 12 weeks] on site, from digging the footings to plastering, depending on size, roof type and how much of the house is opened up. Drawings and approvals come first and can take longer than the build itself. We give you a realistic timescale with your quote.
Want a firm price?
We’ll come and look, measure up and send you a clear written quote.