Are you thinking about extending your home and wondering, “How much will it cost?” You’re not alone. One of the first things homeowners ask us at Hobbs Home Building Co. is about budget. With rising material and labour costs, site-specific issues and local building regulations, understanding the full picture is essential.

Because we specialise in extensions across Devon & Cornwall, we’ve put together this evergreen guide to help you work out a realistic cost, what affects the price, and how to get the most from your build.

1. Average Extension Costs in the UK & South West

Every project is different, but it helps to start with ballpark figures:

  • Single-storey extensions often fall in the range of £1,800–£3,000 per m² (ex. VAT) depending on spec.
  • Double-storey extensions can work out similarly per m² (sometimes slightly less per m²) because you’re spreading foundations and roofing over more space.
  • Simple, lower-spec extensions in some cases can be done nearer £1,500/m², but that usually means more basic finishes.

To make that more tangible:

  • 20m² single-storey extension: roughly £40,000–£56,000
  • 30m² extension: roughly £60,000–£84,000
  • 50m² extension: £100,000+ depending on complexity and glazing

Devon & Cornwall projects can vary depending on access, groundworks and how remote the property is, so always treat these as starting points.

2. What Affects the Cost of an Extension?

There isn’t a “one price fits all” because several elements push the price up or down:

  • Size & shape: the bigger and more complex the footprint, the higher the cost.
  • Specification & finishes: bifolds, rooflights, underfloor heating, high-end kitchens and bathrooms all add cost.
  • Structural requirements: steel beams, removing load-bearing walls, and tying into the existing structure increase labour and engineering costs.
  • Ground conditions & access: poor access, sloping sites, drainage alterations or difficult foundations can add thousands.
  • Professional & statutory costs: architect, structural engineer, planning (if needed), building regulations, and sometimes party wall agreements.
  • External works: patios, steps, retaining walls, and making good the garden are sometimes forgotten but still need to be budgeted for.

3. How to Budget Realistically (and Avoid Hidden Costs)

Here’s a sensible way to approach your budget:

  1. Start with a per m² allowance – for many South West projects, £2,200–£2,800/m² is a sensible working figure for a good-quality single storey.
  2. Add 10–15% contingency – this covers surprises in the ground, spec changes and price fluctuations.
  3. Include professional fees – design, structural, planning/building control.
  4. Decide your finishes early – kitchens, bathrooms and glazing are big-ticket items; choosing late can cause delays and extra cost.
  5. Check VAT and timings – VAT applies to most extensions, and material/labour inflation can affect longer projects.

Our approach at Hobbs Home Building Co. is to walk you through this before we start, so you know what’s included and what would count as an extra.

4. Design & Planning: Get It Right Before You Build

The cheapest time to make changes is before anyone is on site. Good design prevents expensive alterations later.

  • Think about how the new space connects to the existing house.
  • Plan natural light (glazing, rooflights, borrowed light from other rooms).
  • Consider ceiling heights and roof form – these affect the look and the cost.
  • Don’t forget services – electrics, heating, water, ventilation.

See our recent extension projects in Devon & Cornwall for ideas and layouts.

If you want layout or interiors help at this stage, pair this with our interiors arm: Hobbs Home Interiors – designing from the build up makes the whole project run smoother.

5. How Long Does an Extension Take?

Timings vary, but a typical single-storey extension might look like this:

  • Design & approvals: 2–3 months (can be quicker if straightforward)
  • Groundworks & foundations: 2–4 weeks
  • Main structure, roof, windows: 4–8 weeks
  • Internal fit-out & finishes: 4–6 weeks

Overall, many homeowners should allow around 10–18 weeks on site, longer for larger or more complex builds.

6. How to Choose the Right Builder

Because extensions join onto your existing home (and you may be living there during works), the builder you choose matters as much as the price.

  • Look for extension experience – not every builder specialises in tying new work into old.
  • Ask for a clear quote – what’s included, what’s excluded, and how variations are handled.
  • Check communication – you want progress updates, not surprises.
  • Use a local specialist – we know South West ground conditions, local suppliers, and planning quirks.

Hobbs Home Building Co. delivers extensions start to finish, with clear communication and quality control on site.

7. Why Work With Hobbs Home Building Co.

When you work with us, you get:

  • Specialists in extensions & renovations across Devon & Cornwall
  • Transparent pricing and realistic timeframes
  • Support with design through our interiors side if you need it
  • Project managed build so you’re not chasing trades

Get in touch for a no-obligation extension quote and we’ll talk through your plans, budget and timescales.

8. Extension Cost FAQs

How much will a 30m² single-storey extension cost?

If you allow £2,200–£2,800/m² for a good-quality build, you’re looking at roughly £66,000–£84,000, plus contingency, finishes and any external works.

Is it cheaper to extend or move?

By the time you add agent fees, stamp duty and moving costs, extending is often the better way to get the space you want – and a well-designed extension can add value, too.

Do I always need planning permission?

Not always. Some extensions fall under Permitted Development, but you’ll still need Building Regulations approval. We can advise on this when we see your property.

How much contingency should I allow?

We advise at least 10–15% on top of the build cost to keep the project comfortable.

Next Steps

The first step is simply to talk through your ideas and budget. From there, we can tell you what’s realistic, how long it will take, and what the best approach is for your property.

Request your extension quote

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