Remortgaging to Build an Extension

How Much Does an Extension Cost?

Extension costs in the UK vary widely depending on location, size, specification and complexity. As a rough guide, a single-storey rear extension typically costs between £30,000 and £60,000, while a double-storey extension can range from £50,000 to over £100,000. London and the South East tend to be significantly more expensive than other regions.

These figures include building work, but you'll also need to budget for architect fees, planning applications, building regulations inspections and any landscaping or redecoration once the build is complete. Getting detailed quotes before you apply for the extra borrowing will help you request the right amount.

Planning Permission and Building Regulations

Many single-storey rear extensions fall under permitted development rights, meaning you won't need formal planning permission. However, there are strict limits on size, height and position, and permitted development doesn't apply to all properties (flats, listed buildings and conservation areas have different rules).

Regardless of whether you need planning permission, almost all extensions require building regulations approval to ensure the work meets structural, fire safety, insulation and drainage standards. Your builder or architect can advise on what's needed for your specific project.

How Much Value Does an Extension Add?

A well-designed extension typically adds more value than it costs. A single-storey rear extension can add between 5% and 10% to your home's value, while a double-storey extension could add 10% to 20%. The exact uplift depends on your area, the quality of the build and how well the new space integrates with the rest of the home.

Extensions that create a large open-plan kitchen-diner are particularly popular with buyers and tend to deliver the strongest returns. However, it's worth checking that your extended home wouldn't become the most expensive on the street, as this can limit value growth.

Remortgaging to Fund Your Extension

To remortgage for an extension, you'll need sufficient equity and the ability to afford the higher repayments. Lenders view home improvements positively because the work should increase the property's value. You'll typically need to explain what the funds are for, though detailed plans aren't always required at the application stage.

Timing matters. If your current mortgage deal hasn't expired, check your early repayment charges before proceeding. It may be worth waiting until your deal ends or exploring a further advance from your current lender as an alternative.