Quick answer
There is no dependable one-price-fits-all figure for a single-storey extension. Size matters, but foundations, drainage, structure, glazing, services and finish can change the total significantly. A useful budget separates the shell, internal fit-out and professional or statutory costs.
For property-specific advice, speak to the appropriate planning, design or building-control professional and request a clear written scope from your builder.
The main parts of an extension budget
- Design, surveys, structural calculations and approval fees
- Site setup, access, excavation and foundations
- Drainage changes, brickwork, roof and external openings
- Insulation, glazing, plastering, heating and electrics
- Structural steelwork connecting old and new spaces
- Kitchen, flooring, decoration and other final finishes
- A contingency for appropriate unforeseen work
Why price per square metre can mislead
Area-based estimates can help with very early feasibility, but two extensions of the same size may differ greatly. One might have simple access and a modest specification; another may need drainage moved, large steels, extensive glazing and a fitted kitchen.
A small extension can also have a relatively high rate because preliminaries, approvals and service connections do not reduce in direct proportion to floor area.
What should a written quote clarify?
Check whether the quotation defines excavation assumptions, drainage, steelwork, roof covering, windows and doors, insulation, electrics, heating, plastering and waste removal. It should also state whether kitchens, flooring, decoration, professional fees and approval charges are included or excluded.
Our guide to comparing builders’ quotes provides a fuller checklist.
Ways to keep the budget controlled
- Resolve the layout and major specification before quotations are requested.
- Give every builder the same drawings and information.
- Choose standard dimensions and readily available products where suitable.
- Avoid moving drainage or services without a clear benefit.
- Make key selections early enough to avoid delays and rushed substitutions.
- Retain an appropriate contingency rather than spending the full budget on the initial specification.
See our main page for home extensions in Colchester for the wider service.
Related services and guides
Frequently asked questions
Can you give an accurate extension price without drawings?
An early discussion may identify broad cost drivers, but consistent detailed pricing usually requires a defined design and specification.
What is commonly excluded from a shell quote?
Kitchen units, flooring, decoration, specialist glazing, professional fees and landscaping are examples, but every quote differs.
Why do extension quotes vary so much?
Scope interpretation, allowances, exclusions, specification, workload and understanding of site conditions can all cause variation.
Should I keep a contingency?
A sensible contingency helps cover appropriate unforeseen conditions or choices that change during design.
Planning work on your Colchester home?
Essex Builders can discuss the practical building work, arrange a site visit and provide a clear written quote for suitable projects across Colchester and nearby Essex areas.
Request a free quoteCall 07958 355936