Extension cost calculator
An extension is usually the largest project a homeowner takes on, and headline “per square metre” figures leave out a lot: design, structural engineering, building control, the kitchen or bathroom that goes inside, and making good the existing house.
Tell Estimo the type and size of extension you are considering and it will build an itemised estimate for the shell, the fit-out and the fees, so nothing important is missing from your budget.
What affects the cost of an extension
Type of extension
Single storey rear, side return, wraparound and double storey extensions carry very different structural and roofing costs.
Size and shape
Floor area matters, but corners, roof style and glazing complexity matter as well.
Groundwork
Soil type, drains, nearby trees and foundation depth can move the price significantly.
Glazing and openings
Large bi-fold or sliding doors, rooflights and steel beams are often among the biggest single lines.
Fees and approvals
Architect or designer, structural engineer, planning or permitted development checks, building control and possibly a party wall surveyor.
What goes inside
A new kitchen or bathroom inside the extension is a separate cost that is easy to forget.
Before you start
- Budget for professional fees separately from the build itself.
- Check whether your plans fall under permitted development before committing to a design.
- Ask builders to split quotes into shell, fit-out and external works so you can compare fairly.
Common questions
- Does the estimate include professional fees?
- Estimo lists design, engineering and approval costs as separate lines, so you can see them clearly and remove any you have already paid for.
- Can I estimate a kitchen extension?
- Yes. Describe the extension and the kitchen together and Estimo will price both parts.
- How do I compare extension quotes?
- Upload your quotes to Estimo's comparison tool and it will line them up against each other and flag gaps or missing items.
Get a personalised figure
Estimo shows an indicative range and the reasoning behind it. Ranges are a guide, not a quote.