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Start With Your Neighbours

Semi-detached homes share a party wall, and almost every extension touches it in some way. The Party Wall etc. Act 1996 sets out clear duties. If you plan to build onto or into the party wall — cutting in a beam, raising a wall, forming a new opening — you must serve written notice at least two months before work starts. If you are excavating within three metres of a neighbouring structure and deeper than its foundations, or within six metres in certain circumstances, one month's notice applies.

Notice is not a formality. Your neighbour can consent in writing, which keeps things simple, or dissent, which means surveyors are appointed to prepare a party wall award. An award is not a judgement on you: it records how the work will be done, how the neighbour's property will be protected and who pays for what. For a typical rear extension, budget somewhere between £800 and £1,800, usually met by the person carrying out the work.

Talk to your neighbours before the drawings are finalised. Agree where the skips will sit, how scaffolding will be erected, what working hours will apply and how access will be arranged. Most disputes on semi-detached extensions are about communication, not construction.

Permitted Development, Planning and the Semi-Detached Constraint

Many extensions fall under permitted development, but the rules are tighter for semi-detached homes than for detached houses. Broadly, single-storey rear extensions are limited to 3 metres in depth, side extensions must be single storey, no more than 4 metres high and no wider than half the width of the original house, and no extension may project forward of the principal elevation. Materials must be similar in appearance, and extensions must not cover more than half the land around the original house.

Two-storey rear extensions are also constrained: depth is normally capped at 3 metres, the eaves must not be higher than the existing, and no first-floor windows are allowed in a side elevation. Larger rear extensions may be possible through the prior approval process, where the council consults your neighbours — always check the current limits with your local planning authority before committing to a design.

Conservation areas, Article 4 directions and listed status can remove permitted development rights entirely. A lawful development certificate is inexpensive insurance and worth having before you instruct a builder.

Making the Structure Work

Removing the rear wall of a semi-detached house creates a structural change, not just a cosmetic one. The wall above the new opening must be carried, which normally means a steel beam on padstones and, often, steel columns at each end. Bearings matter: aim for at least 150 mm onto a concrete padstone unless your structural engineer specifies otherwise.

• Do not assume a beam size. A 152 x 152 universal column is common, but spans, roof loads and wall weights vary enormously.
• Check whether the flank walls can resist the horizontal thrust of the new frame, or whether a portal frame is needed.
• Consider the existing roof. Older semi-detached roofs sometimes rely on the rear wall for stability and wind bracing.
• Ask the engineer about differential movement between the new extension and the original house.

If the existing rear wall is exposed during the works, it must be insulated to current standards where the thermal line is altered. Getting this wrong shows up as cold bridging and condensation later.

Drainage and Foundations

Before you dig, establish what is underground. A CCTV survey of the drains and a check for public sewers are essential. If a public sewer runs under or within three metres of the extension, you will need a build-over agreement from your sewerage undertaker, sometimes requiring the pipe to be diverted or encased in concrete.

Surface water needs a plan too. Rainwater from the new roof can usually connect to the existing system, but many authorities expect sustainable drainage — a soakaway, permeable paving or a rainwater butt — to limit what leaves the site. Soakaways must be sited well away from foundations and boundaries.

Foundations are typically trench fill at around one metre deep, but on shrinkable clay, near mature trees, or where old services run, they may need to go deeper. Keep finished floor levels at least 150 mm above external ground and never bridge the existing damp-proof course.

Building Regulations: Where the Detail Matters

Building regulations approval is separate from planning permission and is non-negotiable. You can use either your local authority building control team or a registered approved inspector, and you can submit full plans or a building notice. Full plans are usually the better route for an extension.

• Part A covers structure — steelwork calculations and foundation design are checked.
• Part B covers fire. Extensions close to a boundary may need fire-resistant external walls.
• Part L sets energy targets. Expect U-values around 0.18 W/m²K for new walls and floors, and limits on glazing — typically no more than 25% of the extension floor area under the simplified approach.
• Part F requires adequate ventilation, especially if the extension is open to the kitchen.
• Part H deals with drainage, and Part C with damp and ground moisture.

Keep the completion certificate. You will need it when you sell, and its absence causes real problems at conveyancing stage.

Sequencing the Work

A realistic order for a semi-detached extension runs: party wall notices and awards, planning or lawful development confirmation, building regulations approval, structural design, then contractor procurement. Allow two weeks of float — weather, deliveries and neighbour access rarely run to plan. Insist on a written schedule, staged payments tied to milestones, and a clear retention of around 5% released after snagging. Check insurance: your contractor needs public liability cover, and you must tell your own insurer before work begins.

Done properly, an extension to a semi-detached house is one of the most reliable ways to add space and value. The trick is to treat the neighbours, the drains and the paperwork as seriously as the bricks and steel.