South East

Budget calculator

What should you set aside?

An honest range before we've met, based on what similar jobs in South East London have cost us to deliver this year. Pick the project, drag the size, choose a level of finish.

1 · Your range

Pick the job, drag the size, choose a level of finish. The figure updates as you go.

Project
New floor area
25 m²
10 m²60 m²
Level of finish
Estimated range · Rear / side extension
£106,000 – £139,500
Single-storey extension, shell to finish, with glazing and a new kitchen space ready to fit.
—Foundations, steels and roof
—Glazing, doors and rooflights
—Plastering, electrics, heating and decoration

Guide only. Excludes VAT, structural engineer's and planning fees, and anything we find behind the plaster. We'll give you a fixed price after a site visit.

Get this range in writing — book a call
Your figures come with you — we see them before the call.

2 · A real one, costed

What a rear / side extension actually cost.

After: A side return that looks like it was always there
Before: A side return that looks like it was always there
⇔
BeforeAfter
Before and after · drag to compareHerne Hill · January 2026
Worked example · Rear / side extension

A side return that looks like it was always there

£112k incl. kitchen
What this one cost
13 weeks
On site
Victorian terrace
The house

The clients wanted the extension to be invisible from the garden. Not literally, but you shouldn't notice where old meets new.

Where the money went · £112k incl. kitchen
Structure and shell£45k · 40%
Glazing and roof£16k · 14%
Plumbing and electrics£13k · 12%
Kitchen and joinery£27k · 24%
Finishes and decoration£11k · 10%
Read the full project

3 · Whole-project planner

Doing more than one thing?

Loft, extension and kitchen as one programme costs less and finishes sooner than three separate jobs. Tick what you're considering.

Include

Ranges use typical sizes for a South East London terrace and your chosen level of finish above. Fine-tune each job on its own calculator.

Combined estimate · 2 projects
£120,500 – £160,500
Programme14 weeks
One team instead of several≈ £12,000 and 4 weeks
Plan it on a call

4 · Design only

Just want the drawings?

If you already have a builder, Colin's studio can design alone. The fee is credited back against the build if you go ahead with us.

Rooms
3 rooms
1 roomWhole house
Scope
Design fee · Concept + drawings
£2,500 – £4,000
Plans, elevations, lighting and joinery drawings a builder can price from.
—Two design meetings and one round of revisions included
—Drawings issued as PDF and DWG for any builder to price
—Credited in full against a South East Build contract
Book a design consultation

How we price

What moves the number, and what doesn't.

Structure
Steels, foundations and anything that touches the roof move the number most. A knock-through can be a third of a kitchen budget.
Services
Moving a soil stack, a boiler or the consumer unit costs more than people expect. Leaving them where they are is the easiest saving.
Level of finish
Off-the-shelf to made-to-measure roughly doubles the cost of joinery and tiling, but not of the building work behind it.
What we find
Period houses hide things. We carry a 10% contingency in every price and tell you the day we need to touch it.
Access
A terrace with no side access means everything comes through the front door. It adds days, not weeks.
What doesn't move it
Paint colour, handles, taps and lights. Choose freely; these are where taste costs nothing extra.

Further reading

Building costs and VAT: the official sources

A fixed price starts with a twenty-minute call and ends with a survey.
Book the callAsk a question first
WhatsApp a photo of the room — you'll have a rough range by the next working day.
Budget calculator · What should you set aside? · South East Build