Extending a semi where the neighbour has already extended

Your neighbour already extended the other half of the semi, so you assume you can build the same thing. That assumption is half right, and the half that is wrong catches a lot of householders.

Every application is decided on its own merits against the development plan and any other material considerations. There is no rule that a grant next door entitles you to the same. But consistency of decision making is itself a material consideration, and an authority that approved one half of a matched pair last year needs a reason for refusing the mirror image this year. The argument is available. It just has to be made properly.

Whether a neighbour's extension sets a precedent

Planning law does not work on precedent the way the law of contract does. Section 38(6) of the Planning and Compulsory Purchase Act 2004 requires decisions to be made in accordance with the development plan unless material considerations indicate otherwise, and each site is assessed as it is.

The courts have accepted, though, that like cases should be decided alike, and that a decision maker departing from an earlier decision on a comparable proposal should explain why. So a recent approval next door is evidence, not entitlement.

The weight it carries depends on how comparable the two really are. Ask yourself four questions before relying on it.

• How recent is the approval, and has policy changed since?
• Was it granted under the development plan that applies now?
• Is the proposal equivalent in scale, depth, height and position?
• Was it a planning permission, a prior approval, or permitted development needing no decision at all?

That last one matters more than people expect. If the neighbour built under permitted development, there is no decision to be consistent with, and the council never formed a view on it.

What officers do with an unbalanced pair

A semi detached pair is designed as one composition. Extend one half and the pair becomes asymmetric, which is exactly what an officer will have been concerned about when the first application came in.

Once the asymmetry exists, the calculation changes. A matching extension on the second half often restores the balance the street lost, and officers will sometimes say so directly in the report. This is the strongest version of the argument, and it works best where your proposal is a genuine mirror rather than a larger version.

Where it does not work is where the first extension was itself out of keeping. An approval that the council now regards as a mistake will not be repeated willingly, and the report on your application may say that the earlier permission carries limited weight because it predates the current design guidance.

Two storey side extensions are the sharpest case, because a pair of them turns two semis into a short terrace. Many design guides ask for a set in of about a metre from the side boundary at first floor, a set back from the front elevation, and a subordinate roof form, precisely to stop that happening.

Matching, mirroring or deliberately differing

There are three honest design positions and each can be argued.

Mirror the neighbour exactly. Simple, easy to justify on street scene grounds, and usually the fastest route through the process. The risk is inheriting a bad design.

Match the principles but not the detail. Same depth and height, different fenestration or a better roof form. This works where the earlier extension was clumsy but its scale was accepted.

Design something clearly different. Defensible where the two halves have already diverged, where the extension is at the rear and not visible from the street, or where the architecture justifies it. Harder work, and it needs a stronger statement.

What rarely succeeds is a proposal that takes the neighbour's approved depth and adds to it. A precedent argument runs only as far as the thing that was approved, and going beyond it invites the officer to treat the whole scheme as a fresh proposal on its merits. Where the extra depth really matters, the permitted development and prior approval routes may help instead, as set out in our note on rear extension depth limits and the larger home right.

Cumulative effect along the street

Officers look up and down the road rather than at your pair alone.

Where several properties in a run have already extended, the character argument weakens and your case strengthens. Where yours would be the second in an otherwise unaltered street, expect more scrutiny, and expect the report to discuss whether approving it would make it difficult to resist others.

That second point is worth understanding rather than resenting. A council is entitled to consider the effect of granting permission on future decisions, and a proposal that would be acceptable on its own can be refused because the cumulative effect of repeating it along the street would not be.

Photograph the street properly. Both directions, from the pavement opposite, showing the extensions that already exist, with addresses noted. Those photographs do more work in a householder application than almost anything else you can submit.

Amenity is judged against the situation as it is

This is the point that most often surprises applicants, and it usually helps them.

Neighbour amenity is assessed against the existing situation, which includes the neighbour's extension as built. If their two storey side addition already shortens the light to your side windows, the baseline for assessing your proposal includes it.

The practical consequences run both ways. Your extension is measured for its effect on their windows in their current position, which may now be further from your boundary or differently arranged. And where they have brought a habitable room window closer to the boundary as part of their works, that window is part of the baseline and your design has to respect it.

Most councils apply a rule of thumb taken from their own design guidance, commonly a 45 degree line drawn in plan and elevation from the centre of the nearest habitable room window. Check the adopted guidance for your authority, because the angle, the measuring point and whether it applies in plan, elevation or both all vary.

Checking what the neighbour actually got

Do this before you design anything. It takes half an hour on the council's public register.

Look for the application reference, the approved drawings, the decision date, the conditions, and the officer report. The report is the useful document, because it tells you how the authority assessed the same street, the same relationship and often the same neighbour objections.

Then compare the approved drawings against what is actually built. Extensions are frequently built deeper, taller or with different windows than approved, and an argument founded on the built form rather than the permitted form collapses the moment the case officer checks. If the neighbour's extension is unauthorised, relying on it is worse than useless, because it can prompt an enforcement enquiry that helps nobody.

Note any conditions too. A condition removing permitted development rights, or requiring obscure glazing to a side window, tells you what the council was worried about and what it will be worried about again.

Framing the argument in your statement

Put the consistency argument in writing rather than assuming the officer will make it for you.

A short Planning Statement that sets out the neighbouring approval by reference number and date, summarises the officer's reasoning on that application, identifies the ways in which your proposal is equivalent, and addresses any respect in which it differs, is far more persuasive than a covering letter saying that the neighbour has one.

Where the appearance is the issue, the Design and Access Statement is where the street scene analysis and the elevational study belong. Include a drawing of the pair as existing and as proposed, at the same scale, so the restored symmetry is visible rather than described. Materials carry a lot of weight on a semi, and the detail of getting them right is covered in our note on matching materials to the existing house.

Be straight about the differences. An officer who finds an unmentioned difference between your scheme and the one you are relying on will treat the rest of the statement more sceptically.

And if the neighbour supports the application, ask them to write and say so during the consultation period. It is not determinative, but an absence of objection from the property most affected removes the easiest reason to refuse.

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This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.