How Much Does A Home Extension Really Cost?

Grant Redfern
Dec 3, 2025
2 min read

THE HOME EXTENSION SERIES — PART 3

Your step-by-step guide to getting it right from day one.

🌟 INTRO: Why Extension Costs Are So Confusing

If you’ve started researching extension prices online, you’ve probably seen everything from £1,200/m² to £3,500/m².

No wonder homeowners are confused.

The reality? Extension costs vary massively based on:

size
layout
structure
glazing
finishes
complexity
ground conditions

This guide gives you realistic numbers we see every day as a design & build contractor — so you can plan confidently and avoid budget blowouts.

🧱 1. The Basic Numbers (Realistic Cost Per m²)

Let’s cut through the noise.

🏠 Single-storey extension:

£1,800–£2,500 per m² (standard)

£2,500–£3,200+ per m² (high spec / lots of glazing / steels)

🏡 Double-storey extension:

£2,200–£3,000 per m² (realistic build cost)

What pushes costs up?

Large steel beams or goalposts
Complex knock-throughs
High-end glazing (sliders/bifolds)
Flat roofs with skylights/lanterns
Poor ground conditions
Bespoke details
Reconfiguring internal rooms
Kitchens & bathrooms

Remember: m² cost is helpful — but not the full story.

🏗️2. Structural Costs You Should Expect

This is where quotes can jump.

Typical ranges:

Steel beams: £1,500–£15,000+ depending on spans
Goalpost frames: £6,000–£15,000
Box frames: £10,000–£25,000
Foundations: £2,000–£10,000 depending on depth
Drainage reroutes: £1,500–£6,000
Build-over agreements: £700–£1,500 (plus labour)

These aren’t “extras” — they’re common.

🪟 3. Glazing: The Silent Budget Killer

Glass makes a space — but it also makes the cost.

Typical ranges:

Bifold doors: £3,000–£7,000
Sliding doors: £4,000–£10,000
Feature glazing / corners: £5,000–£25,000
Roof lantern: £1,000–£3,000
Skylights: £600–£2,000 each

This is why design decisions matter early — glazing can transform a budget.

🏡 4. Kitchen & Interior Costs

These are usually NOT included in a basic build quote.

Typical realistic numbers:

Kitchen supply: £8,000–£25,000
Kitchen installation: £1,500–£4,000
Flooring: £25–£80 per m²
Heating upgrades: £1,500–£4,000
Decorating: £1,000–£4,000

This is where “£40k extension” myths fall apart. A proper spec takes it much higher.

📑 5. Professional Fees You Need to Budget For

These vary, but here are reliable averages:

Architectural drawings: £1,500–£3,500
Structural engineer: £500–£2,000
Planning application: £206 (government fee)
Building Control: £600–£1,200
Party Wall: £800–£3,000 (if required)

Most homeowners forget at least two of these.

🏗 6. Contingency: Don’t Skip This

Every project needs one.

Recommended:10–15% of the total build budget

This covers:

foundation surprises
drainage
additional steels
material shortages
spec changes you decide on during the build

Skipping contingency is the No.1 cause of stress.

👉 NEXT WEEK:

Part 4 — What Actually Happens During The Build?

A step-by-step look at the construction process from start to finish..