What affects the cost of a house extension in Wiltshire?

Ask ten builders what an extension costs and you will hear ten different per-square-metre figures. They are rarely useful, because two extensions of the same size can differ enormously in price. Here is what actually moves the number, so you can plan a realistic budget and compare quotes properly.

1. What is under the ground

Every extension starts with groundworks, and this is where budgets most often go wrong. Ground conditions vary across Wiltshire. Clay soils, particularly near mature trees, can need deeper or trench-fill foundations. Existing drains running where the extension will sit may need diverting. Poor access for a digger means more work by hand.

A good builder will look at the site, ask about trees and drains, and explain any risk before the dig starts, rather than after.

2. The structure and how it joins the house

A simple single-storey box is the most economical shape. Costs rise with:

• Openings into the existing house. Knocking through to join the extension to the house usually needs a steel beam, padstones and engineer calculations. See our structural alterations page for how this is done.
• Two storeys. More structure, scaffolding and roof, though often better value per square metre of floor space.
• Complex shapes. Corners, angles and changes in level all add labour.

3. The roof

A flat roof is usually simpler than a pitched one, but it needs proper falls, insulation and detailing to last. Pitched roofs that tie into the existing roof need valleys, lead flashings and tiles or slate that match the house. Rooflights and lanterns add cost on top. Our roofing page explains what a well-built extension roof involves.

4. Matching materials

An extension should look like it belongs. On a modern house that may simply mean sourcing the same facing brick. On an older Wiltshire property it can mean reclaimed bricks, Bath stone, and lime mortar mixed to match the existing joints. Matching costs more than standard materials, but it is what makes the finished job look right. Read more on our bricklaying page.

5. Glazing, kitchens and finishes

Large sliding or bi-fold doors, rooflights and high-specification windows are a significant part of many budgets. If the extension is a kitchen or bathroom, the fit-out can cost as much as the shell. Be clear with any builder about which finishes are included in their price and which are yours to supply.

6. Fees, approvals and VAT

Allow for the costs around the build as well as the build itself:

• Architectural drawings and structural engineer calculations
• Planning application fees, if planning permission is needed
• Building regulations fees for plan checking and inspections
• A party wall surveyor, if the work affects a wall or foundations shared with a neighbour
• VAT, which applies to most extension work on existing homes

Our guide to planning permission and building regulations explains the approvals in more detail.

7. A contingency

Even with careful planning, building work uncovers things: an old drain, a soft patch of ground, a lintel that needs replacing once plaster comes off. Holding a contingency in your budget means a surprise does not become a crisis.

How to compare quotes

The cheapest quote is often the one that leaves the most out. When comparing prices, check that each one:

• Is written and itemised, so you can see groundworks, structure, roof and finishes separately
• States what is excluded, such as kitchens, flooring, decorating or skip hire
• Explains how ground conditions will be handled if they differ from expectations
• Sets out a payment schedule tied to stages of work you can see

How we price an extension

At C.E. Bricklaying & Construction we visit, measure and look at the ground and the existing building before giving you a written, itemised quote. Our own team covers groundworks, bricklaying, roofing, carpentry and plastering in-house, with a dedicated architect and structural engineer, so the price covers the whole build rather than one trade.

If you are planning an extension in Devizes or anywhere in Wiltshire, get in touch for a free site visit and quote.