How We Calculate Our Costs

Our calculator and cost guides are built to give homeowners a realistic, all-in budget – not a teaser price. This page explains exactly where our figures come from and how the calculator works, so you can judge how much weight to put on them.

Where our cost data comes from

We do not invent prices. Each rate is cross-checked against several independent, recently published UK sources, then reviewed so that the ranges reflect what homeowners are actually quoted in 2026. Our main sources are:

• HomeOwners Alliance – extension and loft conversion cost guides (2026).
• MyBuilder and MyJobQuote – cost guides based on real jobs and tradesperson quotes (2026).
• BestBuilders – extension cost per m² data by type and specification (2026).
• Homebuilding & Renovating – extension cost guides (2026).
• Checkatrade – average project costs for loft and garage conversions.
• Planning Portal and council fee schedules – planning fees, permitted development rules and building control charges.

We review all figures at least twice a year and whenever a major change occurs, such as new national planning fees.

How the calculator works

1. Building cost

We multiply your floor area by a cost per m² for the project type and finish level you choose. Building cost covers labour and materials from groundworks to decoration, including windows, doors, electrics, plumbing, heating and plastering. It excludes a new kitchen, which you can add separately.

Project type	Basic (£/m²)	Standard (£/m²)	High-end (£/m²)
Single storey / kitchen extension	1,800 – 2,200	2,200 – 2,700	2,700 – 3,500
Double storey extension	1,600 – 1,950	1,950 – 2,400	2,400 – 3,100
Side return extension	2,000 – 2,500	2,500 – 3,100	3,100 – 4,000
Wrap-around extension	1,900 – 2,300	2,300 – 2,900	2,900 – 3,700
Velux loft conversion	610 – 830	700 – 960	870 – 1,170
Dormer loft conversion	1,090 – 1,520	1,220 – 1,740	1,520 – 2,090
Hip to gable loft conversion	1,350 – 1,780	1,480 – 2,000	1,830 – 2,430
Mansard loft conversion	1,480 – 2,090	1,650 – 2,350	2,090 – 2,870
Garage conversion	480 – 660	660 – 920	920 – 1,230

2. Regional adjustment

Labour and material costs vary considerably across the UK. We apply these multipliers to the building cost:

Region	Multiplier
London	1.25
South East	1.12
South West & East of England	1.04
Midlands	0.98
Scotland & Northern Ireland	0.96
North of England & Wales	0.92

3. Professional fees

• Design: 4–7% of the building cost (3–6% for garage conversions).
• Structural engineer: £500–£2,500 (£300–£1,000 for garage conversions).
• Building control: £600–£1,200 (£400–£900 for garage conversions).

4. Planning and party wall

If you tick “needs a planning application” we add the England householder fee of £548; otherwise we add £274 for a lawful development certificate, which we recommend even for permitted development. Party wall surveyors are added at £1,000–£3,000 per neighbour.

5. Kitchen fit-out

For extensions you can add a new kitchen: budget (£8,000–£15,000), mid-range (£15,000–£30,000) or high-end (£30,000–£50,000), including units, worktops, appliances and fitting.

6. VAT and contingency

VAT at 20% is added to everything except the planning fee. Totals are rounded to the nearest £500. We also show a recommended budget with a 10% contingency, because unexpected costs – from poor ground conditions to drainage diversions – are common.

What the calculator cannot know

An online estimate cannot see your house. Your actual price may be higher or lower because of:

• Ground conditions and the depth of foundations needed.
• Access – for example, everything carried through a terraced house.
• Drains, trees and services near the extension.
• Large-format glazing, bespoke joinery or high-spec finishes.
• Your builder’s workload and the time of year.

Use our figures to set a budget and sense-check quotes – then get at least three detailed, itemised quotes from local builders.