I've Just Bought a House and Want to Extend It: A 2026 Homeowner's Guide

You've picked up the keys, the boxes are barely unpacked, and you're already looking at that side return or sloping loft space thinking about what could be. If you've just bought a house and want to extend it, you're in good company: extending rather than moving is one of the most common property decisions in the UK, especially with stamp duty, legal fees and rising mortgage rates making a move to a bigger house expensive. But before you call a builder or start sketching kitchen layouts on the back of an envelope, there's a planning reality check every new homeowner needs to go through first.

The house you've bought comes with its own planning history, its own permitted development status, and possibly restrictions you haven't inherited any paperwork for. What the previous owner could have built is not necessarily what you can build. This guide walks through exactly what to check, what you're likely allowed to do without a full planning application, and how to find out what's already been approved on your street, so you don't waste months and thousands of pounds on a design that was never going to get past the council.

First Things First: What Have You Actually Bought?

Before any conversation about extensions, establish three things about your new home.

Is it in a conservation area, National Park, AONB, the Broads, or a World Heritage Site? These are collectively treated differently under planning law, and they significantly cut back what you can build without permission. If your house sits on this kind of land, side extensions are typically not allowed without planning permission, two-storey rear extensions are ruled out under permitted development, and things like render, timber cladding, pebble dash or tiles on the exterior need an application too. A lot of new buyers don't realise their street is inside a conservation area boundary until they've already commissioned drawings.

Is there an Article 4 direction on the property? Some councils remove specific permitted development rights from an area entirely, usually to protect the character of a street. If Article 4 applies, permitted development rights you'd normally expect (like a modest rear extension) may not exist at all, and everything needs a full application.

Has the house already been extended? This is the one people miss most often. Permitted development rights for extensions are calculated against the "original house" as it stood on 1 July 1948, or as built if constructed after that date, not as it stands today. If a previous owner already built out to the maximum rear extension allowance, your permitted development headroom for a further rear extension may already be used up, even if the current extension looks modest. You inherit the history, not a fresh allowance.

All three of these can be checked before you spend a penny on drawings, and a proper planning report will confirm them against your actual title and local authority records rather than guesswork from Google Street View.

What Extension Can I Build on My Home?

Assuming your house isn't in a conservation area and has no Article 4 restriction, here's what's realistically on the table.

Single-storey rear extensions

This is the bread-and-butter extension for most semis, terraces and detached houses. Under permitted development, you can generally extend:

• Detached houses: up to 8 metres beyond the original rear wall
• Semi-detached and terraced houses: up to 6 metres beyond the original rear wall
• Height capped at 4 metres in both cases

Go beyond the "standard" allowance (which is 4 metres for a detached house or 3 metres for anything else) up to those larger maximums, and you move into the neighbour consultation scheme rather than a straightforward permitted development build. More on that below. None of these larger allowances apply if you're on a conservation area or other article 2(3) land, where the limits drop back down and larger single-storey rear extensions need a full planning application instead.

Wraparound (side and rear combined) extensions

If you're eyeing up an L-shaped extension that fills the side return and continues across the back, both the side and rear rules apply together. It needs to stay single storey, no taller than 4 metres, extend no more than 6 metres beyond the rear wall (8 metres if detached), and the whole extension can't be wider than half the original width of the house. On a conservation area or similar land, that rear projection limit drops to 3 metres (4 metres for a detached house). Go over on width or depth and the whole thing falls outside permitted development, meaning you need consent from the council.

Upward extensions (adding storeys)

Less well known, but genuinely useful for houses that can't extend outward because of small gardens: since 2020 it's been possible to add storeys on top of an existing house under permitted development, in principle. A house with two or more storeys can add up to two additional storeys; a single-storey house can add one. But the rules are tight: the house must have been built between 1 July 1948 and 28 October 2018, the new storeys have to sit on the main part of the house (not a side wing or rear addition), the total height of the extended house can't exceed 18 metres, each new storey can add no more than 3.5 metres, and if your house isn't detached, the new roofline can't be more than 3.5 metres taller than next door's. This route never applies in a conservation area or to a listed building, and even where it's available, it always requires prior approval from the council, so it's never a "just build it" situation.

Loft conversions

A standard loft conversion (adding dormers, raising part of the roof) has its own separate permitted development allowance, but two things catch people out: roof balconies are never permitted development, whatever else applies, and in a conservation area or similar designated land, loft roof extensions of any kind are not permitted development at all. Both routes need a full planning application in those cases.

Front extensions

Keep expectations realistic here. Under permitted development, you generally cannot extend forward of the wall that forms the principal elevation of your house, or forward of a side wall that fronts a highway. This includes the space in front of an imaginary line drawn from the end of that wall to your boundary, so even a modest porch-style addition can fall foul of this if it's in the wrong spot. On a corner plot, there's an extra layer of restriction on the side facing the road too. In practice, almost any extension to the front of a house needs planning permission.

Do I Need Planning Permission for an Extension?

The honest answer: it depends entirely on the size, position and location of what you want to build, and on your specific property's planning history. Many rear and side extensions that fit within the limits above can be built under permitted development, meaning no planning application, but you'll usually still want a Lawful Development Certificate to prove it if you ever sell or remortgage.

Two situations always need a full planning application, regardless of size:

• Anything in a conservation area, National Park, AONB, the Broads or World Heritage Site that goes beyond the reduced allowances (side extensions, multi-storey rear extensions, exterior cladding)
• Anything on a listed building, where permitted development rights for extensions generally don't apply at all

One process that trips up a lot of new owners is the neighbour consultation scheme. If you want a larger single-storey rear extension (beyond 4 metres for a detached house or 3 metres for anything else, up to the larger maximums), you don't get to just start building once you've measured it out. You notify the council, the council writes to your adjoining neighbours, and those neighbours have a window to object. Only once the council confirms no objections need addressing, formally approves it, or 42 days pass without a decision, can work start. This adds real time to your build programme, so if your plans depend on a larger rear extension, factor that six-week window (sometimes longer) into your schedule from day one.

What Extensions Have My Neighbours Had Approved Nearby?

This is one of the most useful, and most underused, steps for anyone who's just bought a house and wants to extend it. Every planning application submitted to a UK local authority is a matter of public record, searchable on your council's planning portal. Before you spend money on architectural drawings, it's worth spending an evening looking at:

• What's already been approved on your own street, particularly on houses of a similar type and age to yours
• Whether extensions of a similar scale to what you're planning were approved under permitted development or needed a full application
• Whether any applications nearby were refused, and why (often overlooking, loss of light, or scale relative to neighbouring properties)
• Whether your specific house has any planning history attached to it already, including any previous extensions, refusals, or enforcement notices

This does several things for you. It gives you a realistic sense of what your council will actually approve versus what the rules technically allow (some councils interpret "impact on amenity" more strictly than others). It flags any local precedent you can reference in a design and access statement if you need one. And it can reveal restrictions specific to your street, like an Article 4 direction or a design code, that don't show up on a general property search.

A planning report pulls this history together properly: your property's planning record, the pattern of approvals and refusals on your street, the article 2(3) status of the land, and any Article 4 restrictions, in one document, rather than you trying to piece it together from a council portal that's often clunky and inconsistently indexed.

The Council Process, Step by Step

Once you know what you're allowed to build, the practical route looks like this:

1. Establish your permitted development position (or confirm you need full planning permission).
2. Get a Lawful Development Certificate if you're relying on permitted development. This isn't legally required to build, but it protects you if a neighbour complains or you sell the house later. Councils typically take around 8 weeks to determine these.
3. If you need planning permission, submit a full application with drawings, a site plan, and (for larger or sensitive sites) a design and access statement. Standard determination time is 8 weeks for most householder applications, though many councils run over this in practice.
4. If the neighbour consultation scheme applies, budget for the 42-day window on top of any other timescales.