Shell Side extension

I want to build a 20 metre squared side extension in the south east of England.
1 window either side and a flat roof.
3.5M RSJ to connect it up to the current house.
i wanted subfloor, hot and cold water pipes along with soil and waste pipe connected up.

What would the price range be for such a job in the south east of England?

Asked by Zakir on 21st Jan 2026
Expert Trade Answers
"The cost for extention in SE range from £2500 per sqm to £3500 sqm labour and Materials"
Answered on 21st Jan 2026 - Member since Jul 2019 - report
"It depends on many factors, age and style of house, local authority, existing heating system and quality of finish. As a rule of thumb current costs are £1500 - £2000 per square meter,"
Answered on 21st Jan 2026 - Member since Feb 2025 - report
"Hi build cost from £2450 per m2 to £3500 per m2 depending in finishes and structure"
Answered on 22nd Jan 2026 - Member since Jul 2025 - report
"For a 20m² side extension in the South East of England, built to shell stage with services included, a realistic price range would be:

£30000– £37000 + VAT

This would typically include:
• Foundations, brick/blockwork
• Flat roof construction
• One window to each side
• Supply and installation of a 3.5m RSJ to connect to the existing house
• Subfloor preparation
• Hot and cold water pipework
• Soil and waste pipe connections

Costs can vary depending on ground conditions, access, specification, and structural complexity, which can only be properly assessed on site."
Answered on 24th Jan 2026 - Member since Sep 2021 - report
"Price range would be between £2500 - £3500 labour and materials"
Answered on 22nd Jan 2026 - Member since Jan 2024 - report
"Hi
That’s a big extension you would be looking at around £35k for the structure then you’ll have internal finishing to pay for
That’s based on Yorkshire area
However you architect should be able to give you a better idea as they would deal with builders in that area"
Answered on 28th Jan 2026 - Member since Apr 2022 - report
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