Single-Storey Extension: The Stages in Order

Before anything is dug

Drawings, structural calculations, building control application, and where the work is close to a shared boundary, party wall notices with the statutory notice period. Drainage searches matter too — building over or near a public sewer needs agreement with the water authority.

This is also when the kitchen or bathroom layout should be broadly settled, because it dictates drainage positions that get built into the slab.

Foundations and drainage

Excavation, inspection of the trench by building control, concrete, then blockwork up to damp-proof course level. Existing drains are diverted or new runs formed, and any manhole inside the footprint is rebuilt with a sealed cover.

Trees, made ground and shrinkable clay all affect foundation depth, and this is the stage where costs most commonly move beyond the original estimate.

Shell, roof and weathertight

Floor slab and insulation, walls up, lintels and any steel to the opening between old and new, then the roof structure and covering, followed by windows and external doors.

The knock-through into the existing house usually happens late, to keep the home usable · Steelwork positions and sizes come from the structural engineer, not the site · Building control inspects at damp-proof course, before the slab is covered, and at other set points · Weathertight is the milestone that makes the rest of the programme predictable

First fix, plaster, second fix

Electrics, plumbing and any underfloor heating go in while the walls are open, then insulation and boarding, then plastering. After drying, second-fix joinery, sanitaryware, sockets and switches, flooring and decoration follow in that order.

Kitchen and bathroom specifications must be confirmed well before first fix, because pipe and cable positions are set then.

Completion

Final building control inspection and completion certificate, electrical certification, any gas safety documentation, and a snagging walk-round. Keep all of it — a buyer's solicitor will ask for the completion certificate.

Frequently asked questions

How long does a single-storey extension take?

It varies with size, ground conditions and specification, and the honest answer comes from a programme against your specific drawings rather than a generic figure.

Can we live in the house during an extension?

Usually yes for most of the build, because the extension is built alongside before the opening is formed. The disruptive periods are the knock-through and any kitchen or bathroom relocation.

When do I need a party wall agreement?

When excavating within defined distances of a neighbouring structure or working on a shared wall. Notices carry statutory notice periods, so they need serving well before the start date.

What causes extension costs to increase mid-build?

Predominantly foundations, because what is under the ground is unknown until it is opened, and client specification changes after first fix. Both are manageable with a contingency and early decisions.