Extending a Terraced House

A terrace gives you the fewest directions to build in and the most people to tell about it. There is a wall in common on each side, a plot that is long and narrow, and no way to get a barrow to the back garden except through the hall. Almost everything that is different about extending this house type comes off those three facts, and the third one is the one that quietly sets the price.

This guide is about how the choice gets made on a terrace: what is available to you, what a previous owner may already have spent, and the order the decisions have to happen in. The pages on each individual job cover how it is built and what it costs.

Which terrace you actually own

One word covers four quite different buildings, and they do not extend the same way at all.

The one most people picture is the byelaw terrace, built roughly between 1875 and 1914: two rooms at the front, a narrower rear wing running back into the garden, and a strip of open ground beside that wing. Solid brickwork, no cavity, footings a couple of courses deep, and a paved yard at the back. It is the only terrace with a side return to fill.

Interwar and post-war terraces built by councils are a different animal. The plan is square, there is no rear wing and therefore no side return, the plots are wider and deeper, and from the 1930s onward the walls are cavity construction. Some rows have a covered passage cut through them every few houses, and whether yours has one is worth knowing before anybody prices access.

Then there are the 1960s and 1970s townhouses, frequently three storeys with an integral garage at the bottom and a trussed roof at the top, where the useful space is the garage rather than the loft. Modern estate terracing from the 1990s onward sits on tighter plots, and the original permission for the estate quite often carried a condition removing permitted development rights from every house on it. Almost nobody checks that, and it is the cheapest thing on this list to find out.

Two neighbours, and sometimes more than two owners

The Party Wall etc. Act 1996 is not part of the planning system, nobody at the council will mention it, and on a terrace it applies on both sides at once.

Three separate things bring the Act in: building on the boundary line, cutting into or altering a party structure, and excavating close to a neighbour’s building and deeper than their foundations go. A full width rear extension on a mid terrace commonly engages the first two against the house on your left, the same two against the house on your right, and the third against whatever sits behind the garden. That is three sets of notices with three clocks running, and the statutory periods are one month or two depending on which limb applies.

Who you serve is a legal question rather than a neighbourly one. An adjoining owner is anybody with an interest in the property greater than a yearly tenancy, so where the house next door is let, the notice goes to the landlord and not to the person who answers the door, and where it has been split into flats there may be several owners for one address. The Act explained in full sets out who has to be served and what happens when somebody dissents.

The practical consequence is a date rather than an argument. Written consent from the adjoining owner is the only thing that shortens a notice period, so serving a fortnight before the groundworkers are booked loses the slot rather than saving time. On a terrace the notices are the first thing that should leave the desk.

The wall itself matters as much as the paperwork. On a byelaw terrace it is frequently a single nine inch skin with the floor joists of both houses bearing in it, so a beam pocket cut from your side is cut into a structure holding somebody else’s bedroom floor up. That gets drawn and calculated rather than decided on the morning.

What “terrace house” means in the rules, and why you count buildings

The permitted development order defines the term, and its definition does not always match the one on your deeds.

For planning purposes a terrace house is a dwellinghouse in a row of three or more buildings which shares a party wall with, or has a main wall adjoining the main wall of, the building on either side. A house at the end of the row counts as well, provided the house beside it meets that test. So an end of terrace is a terrace house, and so is the house at the end of a row of three, even though it looks and feels semi detached from the outside. A pair of two houses is not a terrace at all.

That distinction is worth money in exactly one place. The roof volume allowance is 40 cubic metres for a terrace house and 50 for anything else, so the house at the end of a row of three has ten cubic metres less to work with than the identical looking half of a pair down the street. It is cumulative across everything the roof has already gained, and it does not reset when the house is sold. How the volume allowance is measured works through the arithmetic.

The rear depth limit costs you nothing extra. Class A separates detached houses from everything else rather than separating terraces from semis, so a terrace gets the same three metres of single storey rear extension as the semi next street over, measured from the original rear wall. The prior approval route for a larger home extension takes that to six, subject to a neighbour consultation period which, on a terrace, means the two households with the closest view of it.

The directions available

Back, up, inwards, and on some terraces sideways into a strip you already own.

Backwards is the obvious one and the depth is capped as above. Upwards is the loft, where the volume allowance and the head height under the ridge decide between a rooflight conversion and a dormer on the rear slope. Inwards is reorganising what is already there, the cheapest floor area in the house and the most frequently skipped: a Victorian terrace holds a hall, a front room, a back room and a kitchen, and the walls between them are doing less structural work than people assume.

Sideways only exists on a terrace with a rear wing, where the strip of ground beside it can be built on. It changes those houses more than any other move, because it makes a room wide rather than long, and width is the thing a terrace has never had. How a side return is built covers the structure, the light and the cost of it. What belongs here is the decision: that strip is the only lateral ground the plot will ever have, and once it is built on there is no second go at it.

Adding a whole storey is a fourth option and rarer than the internet suggests. A prior approval route exists for building up over an existing house, but it is off on article 2(3) land, which includes conservation areas, it applies only where the house was constructed between 1 July 1948 and 28 October 2018, and on an attached house the height of the new roof is measured against the roof of the building next door as well as against your own. It still goes to the council, so it is a different application rather than a way round one.

The row moves as one structure

Work at one house in a row can show up at another, which is why the drawings cover walls you do not own.

The party walls are what stiffen a terrace sideways. Your front and rear walls are held in position by them, and taking out a long run of the back wall for full width glazing removes masonry that was doing restraint work as well as carrying load. That is why an engineer specifies restraint straps and sometimes a short return of wall at each end, and why “we can take the whole back off” is a design question rather than a yes.

Chimney breasts are the other one. Removing a breast at ground floor leaves the breast above it, and the stack above that, standing on nothing. On a terrace the stack is frequently shared with next door and sits on the party wall, so the support is designed rather than improvised and the work is notifiable under the Act as well. Whether brackets built into the remaining brickwork will do, or whether it takes a beam, depends on what that wall turns out to be made of.

The wall in the loft usually stops at the ceiling

Get up there with a torch before you plan a room, because this is the item that surprises people.

On a great many pre-war terraces the wall between the two houses stops at ceiling joist level, or continues as a half brick skin with open gaps where the purlins run through. You can often see next door’s rafters. That was normal at the time and it is not what current standards ask for.

Once the loft becomes a habitable room, Building Control will expect that wall to be carried up to the underside of the roof covering and fire stopped where it meets it, so that a fire in one house is not in the next one within minutes. Sound is the other half of the same job and the half you notice daily. Neither is difficult. Both are awkward work in a confined space, they sit on your side of the boundary and therefore in your budget, and they are far cheaper to know about at drawing stage.

Everything goes through the front door

Your builder will price the access constraint. What it should change is your sequence.

With no side gate, the plant available is whatever fits through a standard door opening. A digger that would do a foundation in a day on a semi cannot reach the back, so part of the dig is by hand, concrete arrives by pump or by barrow, and steel comes in bolted sections or over the roof from the street with a crane and a road closure.

Scaffold, structural work and anything involving the ground are the expensive things to repeat. So if the kitchen is coming now and the loft is coming in five years, that is a conversation to have before either is drawn. Add the small logistics early too: a skip on the road needs a permit, a parking bay may need suspending, and the front room becomes a store for several weeks whether you planned it or not.

The drain under the yard is probably not yours

Victorian terraces were drained as a row rather than as a set of houses.

The run across the back of a byelaw terrace typically picks up each house in turn on its way to the street. A pipe serving more than one property is a sewer rather than a private drain, and where it connects to the public network it is the water company’s responsibility. Building over it, or within three metres of it, is settled with the water company through a build over agreement, which is a separate consent from planning and from Building Control and has its own timescale. Lift the covers at both ends of the plot before anybody draws a wall.

Designation, which lands on terraces more than on anything else

The oldest housing sits in the oldest part of a town, and that is where conservation area boundaries were drawn.

Inside one, a side extension generally loses permitted status, cladding needs consent, and the roof enlargement right is withdrawn altogether, so a dormer becomes a full application whatever the volume would have allowed. A rooflight sitting in the plane of the slope falls under a different class and is not removed by the designation on its own. What a conservation area actually changes goes through it properly. An Article 4 direction is a separate instrument, withdrawing specified rights over a defined area and entirely address specific, and it costs nothing to have the council confirm whether one covers your property.

The order this has to happen in

1. Pull the planning history for the address. What a previous owner built has already spent part of your allowance, and on the roof it may have spent all of it.
2. Get into the loft. Look at the roof structure, the head height under the ridge and the state of the wall between you and next door.
3. Lift the drain covers and follow the falls.
4. Measure the front door, the hall and the turn at the bottom of the stairs. That measurement decides what can physically reach the back of your house.
5. Decide the whole plan for the house, including the parts you will not build for years, and only then draw the first phase.
6. Talk to both neighbours yourself, with drawings in your hand, before any formal notice lands on a mat.
7. Then get it priced.

We work through that sequence on a terrace before anything is costed, because on this house type the access answer keeps changing the structural answer and the neighbour answer sets the start date. If you want it done for your own house, send us the address and what you are trying to solve.

The limits and routes described here change from time to time, and how they apply depends on your property, its planning history and any designation over it. Treat this as an explanation of how the decision gets made rather than as confirmation of what you can build.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.