How Much Does a House Extension Cost in Kingston upon Thames?

Typical price ranges in Kingston

For a well built extension in Kingston, Surbiton, New Malden or Norbiton, a sensible starting point is roughly £2,800 to £4,000 per square metre for the building work, before VAT and professional fees. A simple shell with a standard finish sits at the lower end. Large areas of glazing, a complicated roof or a high specification interior push you towards the top or beyond.

As a rough guide to the build cost of common projects:

• Single storey rear extension of 20 to 25 square metres: about £60,000 to £100,000
• Side return infill on a Victorian terrace: about £55,000 to £90,000
• Wraparound extension of 35 to 45 square metres: about £100,000 to £170,000
• Double storey rear extension of 40 to 50 square metres: about £120,000 to £200,000

What pushes the cost up or down

Size matters less than people expect. A small extension still needs foundations, steelwork, a roof, drainage and a site set up, so the cost per square metre is usually higher on a compact side return than on a large rear extension. Adding a second storey is comparatively good value because the foundations and roof are already being paid for.

The other big variables are specification and structure. Removing the whole rear wall for a wide opening means substantial steel and sometimes a steel frame. Bifold or sliding doors can range from £1,500 to over £3,000 per linear metre, and roof lanterns or structural glass add more.

• Ground conditions: much of the area sits on London clay, and nearby trees can mean deeper foundations
• Drainage: building over or near a shared sewer needs a Thames Water build over agreement and sometimes a diversion
• Access: terraced houses with no side access mean materials through the house and slower progress
• Finish: flooring, joinery, lighting and decoration can vary by tens of thousands of pounds

Costs that are often left out of the first figure

A builder's quote normally covers construction only. A new kitchen is the largest extra, typically £15,000 to £50,000 or more depending on the maker and appliances. VAT at 20 per cent applies to most extension work, so check whether the figures you are comparing include it.

You should also allow around 10 to 15 per cent of the build cost for the following, plus a contingency of 10 per cent for anything found once the ground is opened up.

• Architectural drawings and planning or lawful development application to the Royal Borough of Kingston upon Thames
• Structural engineer's calculations
• Building control fees
• Party wall surveyors, commonly £1,000 to £2,000 per neighbour if an award is needed
• Thames Water build over application, where relevant

Planning and local constraints

Many single storey rear extensions can be built under permitted development, which saves time and a planning fee, though a lawful development certificate is worth having for when you sell. Kingston has a number of conservation areas, including parts of Surbiton, Kingston Old Town and Coombe, where permitted development rights can be restricted and materials are scrutinised more closely. That can mean matching brickwork, timber windows or a slate roof, all of which add cost.

Houses close to the Thames or the Hogsmill may fall in a flood zone, which can require a flood risk assessment and raised floor levels or resilient construction. It is worth checking these points before you fix a budget, because they affect the design as well as the price.

How to get a figure you can rely on

Per square metre rates are only good for early budgeting. A dependable price comes from a measured survey, a set of drawings and a written specification that every contractor prices on the same basis. Ask for an itemised quote that states what is excluded, how provisional sums have been calculated, and what the payment stages are.

A design and build route, where one firm handles drawings, approvals and construction, can make it easier to design to a budget from the outset. Whichever route you choose, settle the specification before work starts. Changes made mid build are the most common reason extensions go over budget.