What decides the cost of a single storey extension

Where the money goes

It helps to see the build as separate cost blocks, because that is how quotes are actually built up and where differences between them appear.

Cost blocks in a single storey extension. Block | Scales with size? | Biggest variable. Groundworks and foundations — Partly — Ground conditions, trees, drain runs; Structure and shell — Yes — Steel needed for wide openings; Roof — Yes — Flat, pitched, lantern or rooflights; Glazing and doors — No — Size and type of sliding or bifold doors; Services — Partly — Moving a kitchen, adding heating and drainage; Internal finishes — Yes — Plaster, flooring and decoration standard; Kitchen or bathroom fit out — No — Entirely down to what you choose; Professional fees and approvals — No — Design, structural calculations, building control Rates vary by region, access and specification. We price each job after a site visit rather than applying a standard rate.

The things that quietly increase the number

In our experience these are the items that most often move a budget after the first estimate.

Removing a large section of rear wall, which needs steel and temporary support · Poor ground, high water table or a tree near the foundation line · Drainage runs and inspection chambers in the way of the footprint · Very large glazed openings, which cost far more than a standard door set · Relocating the kitchen rather than extending around the existing one · Party wall matters where the extension sits on or near a boundary

What a quote should show

A quote that gives one figure and a paragraph of description is not comparable with anything. Ask for the work broken into stages, and ask specifically what is excluded.

Common exclusions worth confirming are kitchen units and appliances, floor coverings, decoration, structural engineer fees, building control charges and any unexpected ground works.

Approvals and fees

Many single storey rear extensions fall under permitted development, subject to limits on depth, height and position, with tighter rules in conservation areas and for listed buildings. Whether or not planning permission is needed, building regulations approval is required and comes with fees and inspections.

Structural calculations are usually needed where walls are removed or wide openings formed. These are professional fees rather than build costs, and they belong in the budget from the start.

Keeping the budget under control

The cheapest decisions are the early ones. Keeping the kitchen roughly where it is, choosing a standard sized door set rather than the widest possible opening, and settling the specification before work begins all reduce cost without reducing the space you gain.

Contingency matters too. Ground and drainage are the two things nobody can see before digging, so hold something back for them rather than committing every pound to finishes.

Frequently asked questions

What affects single storey extension cost most?

Ground conditions, the amount of structural steel needed, the size and type of glazing, and whether a kitchen is being relocated. Size matters, but specification often matters more.

Is a per square metre rate reliable for budgeting?

Only as a rough starting point. It ignores fixed costs such as fees and approvals, and it cannot account for glazing choices or difficult ground.

Do I need planning permission for a single storey extension?

Many fall under permitted development within limits on depth, height and position, with tighter rules in designated areas and for listed buildings. Your local authority is the authority on your property.

Does an extension always need building regulations approval?

Yes. Even where planning permission is not required, the work must comply with building regulations and be inspected.

What is usually excluded from an extension quote?

Frequently kitchens and appliances, floor coverings, decoration, engineer fees, building control charges and unforeseen ground works. Ask for exclusions in writing.