Side infill extensions between the bay and the boundary

A side infill between the bay window and the boundary is usually permitted development, but only if it stays single storey, keeps within half the width of the original house, and respects the height limits that apply within two metres of a boundary. Those three limits in Class A of the General Permitted Development Order are what decide whether you need an application. Most side returns on a Victorian or Edwardian terrace comfortably satisfy the width test and fall over on the height one, because a pitched roof tight to a boundary will exceed three metres at the eaves.

Width is measured against the original dwellinghouse, meaning the house as built or as it stood on 1 July 1948. Not as you bought it, and not including an earlier extension.

The half width limit in Class A

Class A permits extensions to a dwellinghouse subject to a list of limitations. For a side extension, the relevant one is that the extension must not have a width greater than half the width of the original dwellinghouse.

Measure the original house across its widest point at the front elevation, excluding any later additions. A terraced house 5.4 metres wide gives you 2.7 metres of permitted side extension width. A side return infill on a typical Victorian terrace is often between 1.8 and 2.4 metres wide, so the width test is rarely the problem.

Two complications arise. Where an earlier side extension exists, its width counts towards the limit, because the limit applies to the cumulative width of side extensions against the original house. And where the house has been extended to the rear, that does not consume the side width allowance, but it does count towards the other cumulative limits in Class A on the proportion of the curtilage covered.

Class A also prohibits a side extension on the principal elevation fronting a highway, and extensions that extend beyond a wall forming a side elevation where the house is on article 2(3) land. In a conservation area, a National Landscape, a National Park or a World Heritage Site, side extensions of more than one storey are not permitted at all, and cladding is removed from the rights, so check whether the house sits in a designated area before anything else. Changes to the external finish raise their own issues, covered in our note on rendering or cladding a house.

Single storey and the height limits at a boundary

A side extension under Class A must be single storey, with a maximum height of four metres overall. That alone does not resolve a side return, because a separate limitation bites much harder.

Where the extension is within two metres of the boundary of the curtilage, the eaves height must not exceed three metres. A side infill is by definition hard against the boundary, so the three metre eaves limit applies, and it applies to the eaves rather than the ridge.

That is the test most side returns fail. A flat roof at 2.9 metres works. A mono-pitch rising from a 2.6 metre eaves to a 3.6 metre abutment at the main house works, because the eaves is the low edge. A gable or hipped roof with eaves at 3.2 metres does not, however modest the ridge.

The practical consequence is that most side infills are designed with a flat or very shallow mono-pitch roof. Where the internal height that produces is unacceptable, you are making a householder planning application, and that is often the better route anyway because it lets you design the roof properly.

Also check the materials limitation: the exterior materials must be of a similar appearance to the existing house. A dark zinc or standing seam roof on a brick terrace is fine on a flat roof that cannot be seen, and arguable where it can.

Rooflights over a side return

A side infill is a deep, narrow space with one external wall, so almost all of its daylight comes from above.

Rooflights in a flat roof are the usual answer, and under Class A they form part of the extension rather than a separate operation, so the four metre and three metre height limits apply to anything that projects. Large rooflights, flat glass walk-on panels and lantern lights all sit above the roof plane, and a lantern on a 2.9 metre flat roof can push the overall height past the three metre eaves limit if the authority treats the lantern as part of the eaves line. The safe reading is to keep the whole assembly below the limits and to check the detail with a building control body in parallel.

Design the daylight rather than defaulting to one lantern. A run of two or three smaller rooflights distributed along the length of the infill gives more even light than a single lantern at one end, which is the usual mistake. Where the infill joins a rear extension, a continuous glazed strip along the new flank wall at high level often does more than the roof glazing.

Overheating is the other side of this. A fully glazed roof on a south or west facing side return will be uncomfortable in summer, and the solution is solar control glazing, external shading or less glass rather than more cooling. Where any raised external terrace or balcony is involved, the privacy rules are different again, as set out in our note on Juliet balconies and overlooking.

Drainage runs under an infill

This is the item that derails more side returns than planning policy does.

The original external rainwater and soil pipes, the gully, and often a shared drain run along exactly the strip you are building over. Since the 2011 transfer of private sewers in England and Wales, a shared drain serving more than one property is likely to be a public sewer, and building over or within three metres of it needs the sewerage undertaker's agreement.

Do three things before the design is fixed. Request the sewer asset plan from the undertaker. Lift the covers and trace the runs, because asset plans are indicative. And where a public sewer crosses the footprint, get the build over agreement application in early, since there is no statutory determination deadline and the undertaker may ask for a lintelled or piled foundation detail.

Internally, the soil stack usually has to move or be rerouted, and the new foul connection has to maintain fall. On a tight infill the floor build-up and the drainage invert levels can conflict, which is a point to resolve on a section drawing rather than on site.

Rainwater from the new roof needs somewhere to go, and many authorities and undertakers now resist connecting new roof area to a combined sewer. A soakaway, a water butt and overflow arrangement, or a green roof all help, and a green roof also slows run-off from the new area.

Party wall implications along the flank

Permitted development says nothing about your neighbour's legal rights, and a side infill sits on the boundary, so the Party Wall etc. Act 1996 is almost always engaged.

Three sections commonly apply. Excavating foundations within three metres of a neighbouring structure and to a lower depth than its foundations requires notice. Work on or against a party fence wall or party wall requires notice. And cutting into a party wall to insert a flashing, a beam or a flitch plate requires notice.

Serve notice properly and early. The statutory notice periods are short but the process of agreeing a party wall award, appointing surveyors and preparing a schedule of condition takes weeks, and starting work without notice exposes you to an injunction. Keep the party wall process entirely separate from the planning question in your own head, because neither one affects the other.

Where the neighbour has already built their own infill, the detail at the junction matters: a shared upstand, a weathering detail and a maintenance access arrangement all need agreement rather than assumption.

Where the infill ties into the rear extension

Most side infills are built at the same time as a rear extension, and the combination is where permitted development limits are most often exceeded.

Check the cumulative tests together. The rear projection limit for a single storey rear extension on a terraced or semi-detached house, the overall four metre height, the three metre eaves within two metres of a boundary, and the limit on the proportion of the original curtilage that may be covered by buildings. A wraparound extension that is permitted development in each direction separately can exceed the curtilage coverage limit when the areas are added.

Where the wraparound takes the scheme outside Class A, a single householder application for the whole thing is simpler than trying to split it. And a lawful development certificate is the only formal way to confirm that works are lawful, so where you are relying on permitted development for part of a larger project, a certificate under section 192 before you start is worth the fee.

Front extensions follow completely different rules and are rarely permitted development at all, as our note on why front extensions are harder than rear ones explains.

Documents for the application

Where you do need an application, a householder submission is short: location plan, block plan, existing and proposed floor plans and elevations at a recognised scale, a section through the new roof showing the junction with the boundary, and the ownership certificate.

A planning statement is not always required for a householder application, but a two-page note dealing with the authority's householder design guidance, the relationship to the neighbour's windows and any 45-degree amenity test earns its place. A design and access statement is required where the site is in a conservation area or the works affect a listed building, and in that case a heritage statement covering the significance of the building and the effect of the works will be expected too. Cladding changes are removed from permitted development in those areas as well.

Enter your postcode on PlanScope to see which documents your authority is likely to ask for before you submit.

This is general guidance, not legal advice. Permitted development limits are detailed and the only formal way to confirm that works are lawful is a lawful development certificate, so check the position with your local planning authority or a qualified professional before relying on it.

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