Project Feasibility Check

What is the Project Feasibility Check?

The Project Feasibility Check is a free eight-question tool for homeowners planning a house extension, loft conversion or garage conversion. It takes two minutes and gives you a property-specific summary covering your most likely planning route, build cost guidance, recent sold prices near you and a recommended design package. You receive the summary by email — no booking is required to get it.

The check replaces any rough estimate you might find on a generic cost calculator. Instead of a single national average, it draws on your project type, your property's planning context and — where available — Hampshire Build's own project data for your area. Where we can't confirm a figure, we say so plainly rather than substituting a number that might not apply to your property.

How extension costs are worked out

Build costs for house extensions are usually quoted in pounds per square metre (£/m²). This covers the structural shell of the extension — groundworks and foundations, walls, roof structure, windows and external doors, insulation and internal plaster finish. It does not include kitchens, bathrooms, fitted joinery, decoration or utility connections. VAT is additional.

In 2026, shell-and-core build costs across Hampshire, Surrey and the surrounding counties typically range from around £2,000 to £3,200 per m² depending on specification and site complexity. A modest single-storey kitchen extension of around 20 m² might cost £40,000–£65,000 to build; a two-storey extension covering 50 m² across both floors might cost £100,000–£160,000. These are indicative ranges — not quotes.

Loft conversions are often quoted as lump sums rather than per m², because most homeowners don't know their loft floor area. A Velux conversion is typically the most affordable; a rear dormer or hip-to-gable involves more structural work and sits at a higher cost range. Garage conversions are usually the most cost-effective way to gain additional habitable space, as the shell already exists.

Planning permission and permitted development — a brief overview

Many single-storey rear extensions qualify as Permitted Development (PD) — a government-granted right that allows the work without a full planning application. Under the current rules, a single-storey rear extension can extend up to 4 metres from the original rear wall of a detached house, or 3 metres for a semi-detached or terraced property, without exceeding 4 metres in height. Side extensions and two-storey extensions nearly always require a full application.

Permitted development rights are removed entirely in conservation areas for certain works, and for all works on listed buildings. They may also be restricted under an Article 4 direction, which some local planning authorities (LPAs) apply to particular streets or housing estates. If your property is a flat or maisonette, PD rights for extensions typically do not apply.

Even where PD applies, we recommend obtaining a Lawful Development Certificate (LDC) from your LPA. This provides written confirmation that your extension is lawful under PD — valuable when you come to sell. An LDC application costs £274 (from 1 April 2026), paid directly to the council. A full householder planning application costs £548.

What affects the cost of an extension?

Counties we serve

Hampshire Build serves homeowners across Hampshire (Winchester, Southampton, Basingstoke, Fareham, Eastleigh, Romsey, Andover, Alton, Fleet, Farnborough, Petersfield), Surrey (Guildford, Woking, Reigate, Farnham, Godalming, Epsom, Esher), Hertfordshire (St Albans, Watford, Harpenden, Hemel Hempstead, Stevenage, Hitchin, Berkhamsted), Berkshire (Reading, Bracknell, Maidenhead, Newbury, Wokingham), Wiltshire (Salisbury, Swindon, Chippenham, Trowbridge), Oxfordshire (Oxford, Abingdon, Didcot, Banbury, Bicester), Buckinghamshire (High Wycombe, Aylesbury, Amersham, Beaconsfield, Milton Keynes) and West Sussex (Chichester, Crawley, Horsham, Haywards Heath, Worthing, Bognor Regis).

If your property is just outside these areas, run the check anyway — we'll confirm on the call whether we can take on your project.

Prefer to talk it through?

Call us Mon–Fri 8:00–18:00, Sat 9:00–16:00. No obligation.