How Much Will My Extension Cost?

How much does a single storey extension cost?

Using the Build It Estimating Service, we’ve priced this 20m2 single-storey rear extension at £34,662 including labour, plant, materials and VAT.

The project features bifold doors and a roof lantern, and could be the perfect choice for creating an open-plan kitchen diner or a similar zone.

An illustration of our benchmark 20m2 flat roof extension

We expect the works to take around eight weeks – so slightly longer than adding a conservatory, plus we’re anticipating an extra cost of just over £11,000.

But the result will be more integrated into the existing house, with a shared heating system and no need to separate the new space with external quality doors.

Benchmark Single Storey Extension Costs

A similar design with a pitched roof would come in slightly more expensive, at around £36,388 including VAT.

This is largely down to the extra costs involved in erecting and finishing a gable wall inside and out. In terms of the covering, while a flat roof structure would cost more to build, surfacing it is significantly cheaper – so the total price comes out around the same.

How much does a double storey extension cost?

Taking on a double-storey addition can give you maximum bang for your buck, adding considerable living space while minimising the potential impact on garden amenity.

An illustration of our 40m2 two-storey pitched roof extension

From a cost perspective, you can think of a two-storey extension as equivalent to stacking a couple of boxes on top of each other. There’s more wall than with a single storey addition, but the amount of roofing and foundations required will be very similar.

Benchmark Two Storey Extension Costs

The Build It Estimating Service gives a total cost of £63,205 for our 40m2 double-storey extension. We’ll be on site for 13 weeks and it will be more difficult to live in the house during the works, so we may need to pay for rental accommodation.

At £1,580 per m2 of extra living space, this model double storey extension project offers better value for money than the flat roof single storey extension. However, the overall cost of a two storey extension is greater.

How much will a basement extension cost?

This benchmark project is a conversion and extension of an existing cellar to create a 33.9m2 zone suitable for a living area, small bathroom and store.

It involves lowering the floor and underpinning the walls to gain sufficient headroom, which swallows up a significant amount of the costs.

Basement Extension Cost

Labour is the single biggest cost, at over 60%, but it’s vital to use a specialist for at least the watertight shell phase – so there’s not a great deal of opportunity to save money with a DIY approach during this phase. While the cost per m2 is high, it could still represent great value in urban centres or where other types of extension aren’t viable.

How much does a loft conversion cost?

Our benchmark loft project is based on a straightforward conversion that creates 40m2 of internal floor area. The prices assume that the roof tiling does not need to be replaced, but do include scaffolding fees.

The rooflight conversion includes the use of seven Velux windows, inserted into the roof, at a cost of around £1,000 each for the labour and materials.

The dormer conversion features three dormer windows (pitched structures built out of the roof plane), with two at the rear and one at the front, plus a single rooflight. This will create extra head height and allows for the inclusion of standard windows (in this case, aluminium).

Main image: This single-storey glazed extension to a Victorian terraced property was designed by IBLA. The new structure has revolutionised the ground floor, transforming it from a dark, underutilised space into a sun-soaked open-plan zone. Photo – Brotherton Lock