Project cost calculator
Get an indicative investment range for your North London extension, loft or basement conversion, renovation or new home in a few minutes. It is an initial guide only — every property, specification and site is different, and a detailed quotation follows a consultation and assessment.
The short answer: a single-storey extension in North London costs £2,800–£4,500 per m² to build (£70,000–£130,000 for a typical 25 m² rear extension); a loft conversion £30,000–£45,000 for a rooflight scheme, £50,000–£75,000 for a rear dormer and £65,000–£95,000 for a hip-to-gable; a whole-house renovation £1,200–£1,800 per m² at a mid-range specification and £1,800–£2,500+ at high specification; a basement £2,800–£6,500+ per m² depending on whether you are converting a cellar, lowering a floor or excavating; and a new-build house £2,400–£4,500 per m² of construction. All exclude VAT and professional fees.
Published “from £35,000” figures are usually a shell before VAT, fees, the kitchen and ground conditions. Quotations from different builders also differ in what they include (preliminaries, contingency, making good, decoration) and in the quality of what they allow for. When comparing, ask for the specification behind the number and whether VAT is included.
Have the site looked at before drawings are commissioned. A feasibility visit that considers the ground, the structure, the drainage and the planning route turns the ranges above into an estimate you can plan around; our process explains the stages. The cost guides linked below go deeper into each project type.
No. It gives an indicative range based on the characteristics of your project and typical North London construction costs at the time of calculation. A quotation requires a site visit, drawings, a specification and, for larger projects, a structural engineer's input. We provide a written fixed-price proposal after that work.
As a September 2026 guide, a single-storey extension in North London costs £2,800–£4,500 per square metre to build, so a 25 m² rear extension is typically £70,000–£130,000 before VAT, professional fees and the kitchen. Double-storey extensions of 40–60 m² are commonly £120,000–£220,000. The calculator adjusts for type, size, finish, features and location.
Specification (kitchens, bathrooms, glazing, joinery, finishes), structural work (steel frames, removing chimney breasts, underpinning), ground conditions and trees (foundation depth), the condition of the existing property, access and parking, and whether the house is lived in during the work. Professional fees add roughly 8–12% and VAT 20% on most work.
The headline range is construction cost excluding VAT and professional fees, which is how builders usually quote. The results screen also shows an all-in figure adding around 10% for design, engineering and approvals and 20% VAT (new-build dwellings are normally zero-rated for VAT).
Because the biggest cost drivers — specification, structure and site conditions — cannot be known from a questionnaire. The range narrows as the project becomes defined: it is wider if you are not sure of the size or are just researching, and tighter once drawings exist. A feasibility visit turns a range into an estimate.
Yes. Inner London postcodes carry higher labour, parking and access costs, and the calculator applies a modest uplift. Our base assumptions are for outer North London and south Hertfordshire, where most of our work is.
We email you a copy of the range and Stoyan Milkov calls to discuss the project — usually within one working day. If it is a good fit we arrange a site visit, normally within a week, and follow up with a written proposal. There is no charge or obligation.
Published 2026 London cost guides from architects, cost consultants and builders (Architecture for London, Checkatrade, houseUP, BuildTeam and others), cross-checked against Milkov & Son's own recent projects. The full model, sources and assumptions are documented and reviewed regularly; figures were last researched in September 2026.
Read next
Extensions
A single-storey extension in London costs £2,800–£4,500 per m² in 2026, so a 25 m² rear extension is typically £70,000–£130,000 before VAT, fees and the kitchen. What moves the number, and what to budget.
3 September 2026 · 8 min read
Renovations
A full house renovation in London costs £1,200–£1,800 per m² for mid-range work and £1,800–£2,500+ for high specification in 2026, so a 110 m² three-bed is £130,000–£200,000 plus VAT and fees. What is included, what moves it, and how to budget.
3 September 2026 · 8 min read
Loft Conversions
A London loft conversion costs £30,000–£45,000 for a rooflight scheme, £50,000–£75,000 for a rear dormer, £65,000–£95,000 for hip-to-gable and £80,000–£130,000+ for a mansard in 2026. The eight factors that decide where yours falls.
22 October 2025 · 7 min read
Rear, side return, wraparound, double storey and kitchen extensions — designed, approved and built by one team.
Whole-house refurbishments and reconfigurations of 1930s, Edwardian and Victorian homes, managed as one project.
Dormer, hip-to-gable, L-shaped and mansard conversions that add a bedroom and bathroom without touching the garden.
Converting cellars, lowering floors and excavating new basements: structure, waterproofing and light designed together.
Discuss your project
Tell us about the property and what you want to achieve. Stoyan reads every enquiry and replies within one working day; site visits are usually within a week.
What happens next: a short call, a site visit if it is a good fit, then a written proposal. No charge, no obligation.
