Planning an extension: what actually happens, in order.

Almost every extension brief starts with a size and works backwards. It is the wrong way round. The useful question is what the room has to do — cook, eat, work, hold a sofa, catch the afternoon sun — because that determines the shape, the glazing orientation and where the doors go far more than the square metres do.

It also decides how the rest of the house works afterwards. The commonest regret on a single-storey rear extension is not that it was too small; it is that the original back room became a dark corridor to it.

Many single-storey rear extensions on houses fall under permitted development rights, which means no planning application — but the rights come with limits on depth, height, eaves height, materials and how much of the garden is built on, and they are narrower on a semi than a detached, narrower again if the house has already been extended, and narrower still in a conservation area or on a listed building.

Chester has a lot of all three. If the house is in a conservation area, or is listed, or has had permitted development rights removed by an Article 4 direction, the calculation changes completely. And permitted development is not a nothing-to-do route either — a Lawful Development Certificate is worth having, because it is the piece of paper a buyer’s solicitor will ask for in ten years’ time.

Planning permission is about whether you may build it. Building control is about whether it is built properly — foundations, structure, insulation, fire safety, ventilation, drainage, glazing and electrical work. An extension needs building regulations approval whether or not it needed planning permission. The two are unrelated and people conflate them constantly.

In practice that means inspections at set stages: foundations before they are covered, damp-proof course and oversite, structural elements, insulation, drainage before it is backfilled, and completion. Missing an inspection is expensive, because the only remedy is opening the work up again.

If you are excavating within three metres of a neighbour’s structure and deeper than their foundations, building on the line of junction, or cutting into a shared wall, the Party Wall etc. Act 1996 applies and notices have to be served — usually two months before work starts on a party wall, one month for excavation and line-of-junction work.

It is not a hostile process and it is not optional. Served early and explained properly, most neighbours simply consent. Served late, or not at all, it becomes the single most effective way to stop a job dead.

Where the new extension meets the old house, something has to carry the wall you are removing. That means a steel beam sized by a structural engineer, temporary props and needles while the opening is formed, and padstones that can actually take the load at each end.

This is the first heavy stage and everything downstream waits on it — you cannot board, plaster, run services or set out a kitchen until the opening is formed and the structure is permanent. Get the opening width settled at this point, with the finished room in mind, because widening it later means doing the whole thing twice.

Roof and weathering-in, then windows and doors, and only then does the inside start: first-fix electrics and plumbing, insulation, boarding, plaster, screed or floor build-up, second-fix joinery, tiling, decoration, flooring, and finally the garden put back.

Every one of those has to follow the one before it, and every gap between them is a week added. This is the whole argument for one company with its own trades: the sequence is a programme rather than a negotiation between six firms’ diaries.