• As a general rule, many house extensions cost £1,800–£3,000 per m² for a standard-quality single-storey extension. 

• For a 20 m² extension the typical cost is roughly £40,000–£56,000. 

• A 30 m² extension can cost around £60,000–£84,000, depending on finish and complexity. 

• For larger extensions (e.g., 50 m²) costs often sit in the £100,000–£140,000 range. 

• Two-storey extensions can be more cost-efficient per m² (because foundations/roof/groundworks get shared), though the total cost will still be higher than a small single-storey. 

What causes variation?

• The size and shape of the extension.

• The finish level (basic shell vs full fitted kitchen/bathroom, high-spec glazing, insulation, floor finishes, etc.). 

• Site-specific factors: ground conditions, access, whether existing foundations need strengthening or structural work is needed. 

• Additional features: plumbing, electrics, heating, flooring, doors/windows, and internal finishes add considerable cost beyond the structural shell.

You may be able to extend without full planning permission under “permitted development rights” — but it depends on the nature of the extension (house type, size, orientation). Key general rules:

• In many cases a single-storey rear extension can be built under permitted development within size limits. For example, adding a modest depth rear extension may be possible without formal planning permission. 

• However, if you exceed size/depth limits, or if your property sits in a conservation area, is listed, or subject to restrictions, you may need full planning permission. 

• Because the rules vary by property type (semi, detached, terraced), location and local authority, you should check with your local planning authority before assuming you can build without permission.

In short: you can extend without permission — but only within specified permitted-development limits which must be carefully checked for your exact property.

If you’re trying to maximise additional space on a budget, here are commonly the most cost-effective approaches:

• Convert an existing space (garage, loft, basement) rather than build a full ground-up extension — cheaper because you avoid major structural works and groundwork.

• Keep it small and simple: a small single-storey extension with a basic finish (shell plus minimal internal finishes) keeps per-m² and total costs lower.

• Limit fittings and high-spec finishes: save money by delaying or using simpler windows/doors, standard flooring, and minimal decoration.

• Phase the build: first build a shell, later fit out interiors; that spreads cost and lets you control spend.

• Use cost-effective construction methods: simple rectangular shape, avoiding complex rooflines or large open-span structures, reduces foundations/structural costs and minimises waste.

In many cases, a modest garage conversion or small single-storey side/ rear extension is among the cheapest ways to add extra living space.