How to plan a house extension that gets approved

An extension is the most common way to add space to a UK home, and the most common place for a project to stall. The difference between a smooth approval and months of delay usually comes down to two things: confirming the route before you design, and submitting drawings that answer the officer's questions rather than raising them.

Start by working out whether your extension falls under permitted development or needs a full planning application. Many single-storey rear extensions sit within permitted development limits, while larger, two-storey or front extensions usually need full planning. Getting this clear at the outset saves you from designing something that was never going to be approved.

Next comes the drawing set. A council-ready package typically includes existing and proposed floor plans, elevations, a site location plan and a block plan. These show the council exactly what is there now and what you intend to build, drawn accurately and to scale.

A few things commonly trip people up: exceeding depth or height limits, building too close to a boundary, or forgetting that a previous extension may have used up part of your allowance. It is worth checking these before you commit to a design.

At Exp|Arc we confirm your route on day one, quote a fixed package, and revise the drawings until they are right. If an application is needed, we can prepare and manage the submission for you.

The limits that actually matter

Under permitted development a single-storey rear extension is generally limited to 3m on a terraced or semi-detached house and 4m on a detached one, with a 4m height cap. The larger home extension route pushes that to 6m and 8m through prior approval, where the council notifies your neighbours and considers any objections. Height within 2m of a boundary drops to 3m at the eaves, which is what catches out narrow plots.

Extensions are measured from the original rear wall, not the current one
Side extensions are limited to half the width of the original house
Front extensions almost always need a full application
Conservation areas and Article 4 directions remove rights entirely

Why applications get refused

Refusals rarely come from the idea itself. They come from overlooking neighbouring windows, loss of light, a design that ignores the character of the street, or drawings that leave the officer guessing. Around 88% of householder applications succeed nationally, so a refusal usually signals a preparation problem rather than an impossible project.

What it costs, end to end

Budget for three separate things: the drawings, the council fee, and a structural engineer if you are opening up walls. Planning drawings typically run £1,200 to £2,500, a full set with Building Regulations £2,500 to £5,000, the householder application fee is £548 in England from April 2026, and an engineer £700 to £2,000. Anyone quoting a percentage of build cost is quoting a number that will move.

A sensible order of work

Confirm the route, then design, then submit, then produce the technical set. Doing it the other way round, designing first and checking later, is the single most common way people lose money on an extension.

Where we come in

We confirm which route applies to your property in writing, prepare the drawings, and handle the application with the council. See extension drawings, or get your drawings quoted with no obligation. If any application we submit is refused, you get a 100% refund within 5 days.