Rear extension on a 1930s semi in a conservation area

This conservation area extension application example is illustrative. It is a composite built from the way these applications normally run, not a record of a real address, a real council or a real decision, and the figures in it are there to show the method rather than to be relied on.

The scenario: a 1930s semi-detached house in a conservation area covering an inter-war garden suburb. The owners wanted a single storey rear extension 3.5 metres deep across the full width of the house, 3 metres high to a flat roof with a lantern, replacing a tired lean-to. The useful part of the story is why permitted development was not available, because the reason is not the one most people assume.

The site and the constraint picture

Five constraints applied, and all five were established before anything was drawn.

• Conservation area, which makes the property article 2(3) land for permitted development purposes.
• An article 4 direction covering the suburb, removing Class A, B and C householder rights.
• No listing, and no listed building within a distance that raised a setting issue.
• A category B silver birch in the rear garden, not protected by an Order but within the conservation area, so subject to the six week notice requirement for works to trees.
• Flood Zone 1, so no flood risk assessment required.

The curtilage coverage calculation also mattered. Class A limits buildings in the curtilage, excluding the original house, to 50 per cent of the curtilage area after the works. With an existing detached garage and a shed, the plot was close to that figure before the extension was added.

Why permitted development was not available

The common assumption is that conservation area designation removes the right to a rear extension. It does not. What article 2(3) status removes from Class A is cladding, single storey side extensions, rear extensions of more than one storey, and the larger rear extension allowances that would otherwise be available through the prior approval route.

A single storey rear extension within the basic limits, meaning no more than 3 metres beyond the original rear wall for an attached house and no more than 4 metres high, remains permitted development in a conservation area unless something else removes it.

Three things removed it here.

1. The article 4 direction, which withdrew Class A entirely for this area. That was decisive on its own.
2. The depth. At 3.5 metres the extension exceeded the 3 metre allowance for an attached house, and the larger allowance through prior approval is not available on article 2(3) land.
3. The curtilage coverage, which the garage and shed had largely used.

The lesson from the first point is procedural. Article 4 directions are published by the authority and are not always obvious from a map search, so check the authority's own article 4 register for the address before assuming anything. Where there is doubt, a certificate under section 192 of the Town and Country Planning Act 1990 is the only formal confirmation that works would be lawful without permission.

The drawings that went in

A householder application is a short pack, and the drawings carried most of the weight.

Two annotations did disproportionate work. The original rear wall was labelled on both the existing and proposed plans, which removed any argument about where the 3.5 metres was measured from. And the root protection area was shown as a hatched zone with the proposed foundation type noted inside it.

How the heritage appraisal was structured

Section 72 of the Planning (Listed Buildings and Conservation Areas) Act 1990 requires special attention to the desirability of preserving or enhancing the character or appearance of a conservation area. The Heritage Statement was written to answer that duty in five steps rather than to describe the house.

First, identifying the asset: the conservation area itself, with the house as an unlisted building that contributes to its character through its form, materials and the rhythm of the pairs along the street.

Second, describing significance by value. The architectural value lay in the consistency of the inter-war pairs, the hipped roofs, the tile-hung bays and the brick. Historic value lay in the suburb's planned layout. The appraisal cited the authority's own conservation area appraisal for the defining characteristics rather than asserting them.

Third, assessing the contribution of the parts affected. The rear elevation was altered, half-rendered and carried a 1970s lean-to. The appraisal said so plainly: the part of the building being changed made a limited contribution.

Fourth, assessing the effect. A single storey extension across a rear elevation that is not visible from any public vantage point, replacing an addition of no merit, with brick to match and a flat roof set below the first floor cill line.

Fifth, the conclusion. The appraisal concluded that the character and appearance of the area would be preserved, so no harm arose and no public benefit balance was needed. Where an appraisal cannot reach that conclusion, it has to identify the level of harm and then address the policy balance, and saying so openly reads better than a conclusion that nobody believes.

The amenity evidence

The neighbour's situation was the real risk, because a semi-detached pair shares a boundary and the neighbour's kitchen window sat close to it.

Three pieces of evidence went in.

A plan showing the 45 degree line taken from the midpoint of the neighbour's nearest habitable room window, following the version in the authority's householder design guide. The extension sat outside it.

A section showing the 3 metre flat roof against the existing 1.8 metre boundary fence, with the shadow angle at the March equinox marked.

A note on overlooking: single storey, no first floor openings, no roof terrace, so no new intervisibility. The lantern was shown as fixed and set back from the boundary.

The Planning Statement pulled these together with the heritage conclusion, the curtilage coverage calculation and a paragraph on the birch. It ran to four pages.

One procedural point became live after permission. Work to the shared flank wall engaged the party wall procedure, which runs on its own timetable and has nothing to do with the planning permission. That sequencing catches people out, as a party wall notice that stopped a rear extension programme shows.

The conditions, and how they were discharged

The decision was delegated, with four conditions.

Development in accordance with the approved plans, listed by drawing number and revision. Nothing to discharge, but it fixes the scheme, so any later change needs a formal amendment.

Materials to match the existing dwelling, with samples to be submitted and approved before the walls are built. Discharged with photographs of a sample panel, the brick manufacturer's specification and a mortar mix note. The sample panel is worth building before you apply, because it answers the question in advance.

Works to be carried out in accordance with an arboricultural method statement and tree protection plan. The Arboricultural Impact Assessment specified a no-dig foundation within the root protection area, with protective fencing erected before any machinery arrived. Discharged with the method statement and dated photographs of the fencing in position.

No windows or other openings in the flank elevation without further permission. Nothing to discharge, but it limits future options and the owners were told so.

What would be done differently

Four things, and they are the four that recur.

Check the article 4 register at the first conversation rather than after the measured survey. The direction changed the project from a prior approval exercise into a full application, and it would have changed the fee and programme advice given at the outset.

Survey the tree before designing the foundations. Redesigning a foundation inside a root protection area after the layout is fixed costs more than commissioning the survey at the start.

Measure the curtilage coverage early. On plots with a garage and outbuildings, the 50 per cent figure is often the binding constraint and nobody checks it until late.

And show the neighbour the drawings before submission. The objection that arrived was about construction noise rather than about the extension, and a conversation beforehand would probably have avoided it. Neighbour comments do not bind the authority, but an unanswered objection makes a delegated decision slower.

Browse all 66 PlanScope report types to see which documents a householder application on your own site is likely to need.

This is general guidance, not legal advice, and the scenario above is illustrative rather than a record of a real application. 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.