Planning a House Extension: The Steps in the Right Order

Extensions rarely go wrong at the building stage; they go wrong months earlier, in the ordering of decisions. Design before budget, builder before drawings, neighbours told last: each wrong order costs a season. This is the sequence we run projects through, and the reasoning behind each step, so you can hold anyone you hire to it.

Start with the two numbers that discipline everything: what the space needs to do, and what you can spend, including VAT, professional fees and a contingency you do not touch for fun. As a realistic London anchor from our own priced projects: new high-standard extension space generally runs 2,400 to 2,800 pounds per square metre before VAT and fees, single storey rear extensions with a proper kitchen fit-out commonly land six figures, and a complete single storey project runs from around 100,000 pounds. If the brief and the budget do not meet, now is the cheap moment to reconcile them, and The Estimator on this site will give you a postcode-specific range in minutes for exactly this conversation.

Good design happens inside real constraints, so the planning strategy belongs in the first design conversation, not after it. Some rear extensions proceed under permitted development; larger schemes and most side returns in conservation areas need an application, and a householder application typically takes around eight weeks once validated. The decision that shapes everything else is scope: designing the extension together with the kitchen, structure and services it exists to house, so one coherent scheme goes to planning, not a shell with question marks inside it.

While planning runs, start the conversations next door. Most London extensions engage the Party Wall Act, through shared walls, boundary walls or excavation near a neighbour's foundations, and the Act runs on real clocks: two months' notice for work to a party structure, one month for boundary walls and notifiable excavation, fourteen days for a response before the surveyor process begins. Served early, these clocks run concurrently with design and pricing and cost you nothing. Discovered late, they stand alone at the front of your build like a roadblock.

Quotes are only as good as the information they price, which is why a proper technical package precedes a proper price. Then compare quotations line by line on identical scope, and put the winner into a written contract with a payment schedule tied to completed stages. This step is where most bad projects were born, in a handshake over an optimistic single-page quote, and it is the step where a design and build company earns its keep: the people who drew it price it, and single-point accountability starts before the skip arrives.

A well-run domestic extension has a rhythm you can see: a programme on paper, one person who runs your site and answers your messages, decisions requested ahead of need rather than shouted up from a trench, and changes priced in writing before they happen, not argued afterwards. Expect a single storey extension to take roughly three to four months on site once it starts in earnest, longer with structural complexity or a full kitchen fit-out. The quality of your February site meetings is decided by the quality of your April paperwork.

After hundreds of these conversations, the failures cluster in two places: budgets set from hope rather than priced space, and sequences run backwards, with builders chosen before drawings exist or neighbours informed after machines arrive. Both are free to avoid. Start with an honest number, run the steps in order, and the build itself becomes the calm part. If you are at step one, price your project with The Estimator today; if you are ready for the steps after it, talk to us.