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One fixed price for the whole job — not the hours

Single-Storey Extension Fair Price

Typical trade: builder. Move the sliders and answer the questions.

Describe the job or snap a photo — AI fills in the sizes

Roof type
How finished?
Bi-fold / sliding doors?
How easy is access and parking?

Fair fixed price for the whole job

£60,450

Fair range: £53,196 – £71,331

20 m² extension

Realistic time on site

35.6 days

Crew / total hours

3 · 800.0 h

Labour (800.0 h × £40)
£32,000
Materials
£23,500
Architect drawings & structural engineer
£2,800
Building control fees
£950
Skips over the build
£1,200

Over £89,164? Ask why in writing.

  • Weekly 'day rate' payments with no fixed scope
  • Asking for 30%+ upfront
  • No building-control sign-off

Insist on

  • One written fixed price for the full scope — no open-ended hours.
  • Deposit of 10–20% at most; balance after you inspect the work.
  • Any extra work agreed and priced in writing before it starts.
  • Agree staged payments tied to milestones (foundations, watertight, first fix, completion) — never weekly payments.
  • Typical programme: 8–12 weeks for 20 m², 12–16 weeks for 40 m²+.

Estimate for an experienced, insured tradesperson. Local prices vary — adjust the hourly rate for your area.

Calcurly.com area calculator showing dimensions, result, and cost breakdown

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About this calculator

Realistic build time and whole-job price for a single-storey rear or side extension. Answer a few questions, move the sliders, and get a brutally realistic time on site plus one fixed price for the whole job — so you can insist on a total rather than open-ended hours. Tradespeople can use it to quote fairly.

Frequently asked questions

How long should this job really take?
The calculator shows realistic on-site hours for an experienced, insured tradesperson working at a normal pace. Quotes far above the upper range deserve a written explanation.
Should I pay by the hour or a fixed price?
Agree a fixed written price for a clearly defined scope. Pay a small deposit (10–20% at most), staged payments on larger jobs, and the balance only after you have inspected the finished work.
What does the Single-Storey Extension Fair Price do?
Realistic build time and whole-job price for a single-storey rear or side extension. Realistic build time and whole-job price for a single-storey rear or side extension. Answer a few questions, move the sliders, and get a brutally realistic time on site plus one fixed price for the whole job — so you can insist on a total rather than open-ended hours. Tradespeople can use it to quote fairly.
Is the Single-Storey Extension Fair Price free to use?
Yes. Every calculator on Calcurly.com is free, works in your browser without sign-up, and can be embedded on your own website.
How accurate is the Single-Storey Extension Fair Price?
Results are calculated in real time using industry-standard formulas. Treat the output as a well-informed estimate — always cross-check with local regulations, product datasheets, or a qualified fair price home jobs professional before committing to a purchase or a job.
Can I embed the Single-Storey Extension Fair Price on my own website?
Yes — copy the iframe snippet at the bottom of this page, or use the Embed Generator to customise the size. The embedded version stays in sync with the live calculator on Calcurly.com.
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