House Extension Cost Calculator
Get a realistic house extension cost estimate in under a minute — based on real 2026 build costs from an FMB-listed builder. No email required.
Calculate Your Extension Budget
Answer four quick questions and get an instant cost range for your extension. Every figure is drawn from real 2026 build costs on projects Crawford Construction has priced and completed — no email address required, no sales call unless you ask for one.
How Does This House Extension Cost Calculator Work?
The calculator multiplies your floor area by a cost-per-square-metre rate, then adjusts for extension type, finish level and location. The base rates come from our own 2026 project pricing: single-storey house extensions in Oxford currently run at £2,800–£3,500 per square metre, with the lower end reflecting straightforward rear extensions on level sites and the upper end covering complex builds — awkward access, structural steelwork, or high-specification glazing.
Oxford Home Extension Case Study
A typical single-storey rear extension on a level garden site in Oxford, using high-spec glazing and matching clay brickwork. We manage the design, structural engineering, and finish alignment from day one.
Double-storey extensions work out cheaper per square metre than you might expect, because the roof and foundations are shared across two floors, but the total spend is naturally higher and the build takes longer — 14–18 weeks against 10–14 for a single storey.
What the calculator can't see is your actual site. It doesn't know whether your garden slopes, whether there's a Victorian drain running exactly where your foundations need to go, or whether your neighbour's extension means a Party Wall etc. Act 1996 award is needed before a spade touches the ground. That's why we treat calculator figures as a starting range, and why every Crawford quote is itemised and fixed-price after a site visit — no vague estimates, no surprises at week six.
What Affects the Cost of a House Extension?
Beyond size and finish, four factors move the number most.
Planning and permissions
Many rear extensions fall under Permitted Development rights and Article 4 Directions, but if you live in a conservation area — North Oxford, Jericho and parts of Headington are common examples — an Article 4 Direction may remove those rights, meaning a full application to Oxford City Council (or Cherwell, West Oxfordshire, South Oxfordshire or Vale of White Horse District Councils, depending on where you are). We handle planning permission, Building Regulations submissions, structural calculations and party wall notices in-house, so the fees are known upfront.
The existing structure
Victorian and Edwardian terraces are wonderful to extend but rarely simple. Shallow foundations, solid walls and shared drains all add cost. On a Victorian end-of-terrace on James Street in East Oxford, we combined a rear extension with a basement and loft conversion costs — a project where careful structural design at the start saved the client significantly against tackling each element piecemeal.
Glazing and steel
Large sliding doors, roof lanterns and open-plan layouts requiring steel beams are the fastest way to move a project from the bottom of the range to the top.
Groundworks
Sloping sites, tree roots and rerouted drainage are the classic unknowns. A site visit catches them before they become variations on your invoice.
Multi-storey Home Expansion
Double-storey extensions and integrated loft conversions completed in Cowley, Oxford. This comprehensive structural approach provides maximum floor area efficiency with coordinated council approvals.
How Accurate Is an Online Extension Cost Estimate?
Honest answer: accurate enough to plan, not accurate enough to build. A well-built calculator using current regional rates — like this one — should land within 15–20% of a final quote for a straightforward project. The gap closes to zero only when a builder has stood in your garden, checked the drains and priced every line item.
That's what our free site consultation and itemised fixed-price quote are for, backed by FMB listing, TrustMark approval, a 10-year workmanship guarantee and staged payments tied to completed milestones — you never hand over large sums upfront.
Ready for a Real Number?
Use the house extension cost calculator above to set your budget range, then let us turn it into a fixed figure. Book a free site consultation with James Crawford, our Project Director, and you'll receive an itemised fixed-price quote — every line explained in plain English.
In-Person Site Consultations
Our Project Director, James Crawford, conducts on-site surveys throughout Oxfordshire to identify ground conditions, drain routes, and boundary access before drafting a final fixed-price schedule.
Get in touch with our team:
Frequently Asked Questions
Detailed answers regarding build costs, fees, timescales, and structural allowances.
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