Materials that match the existing house

The materials similar in appearance planning test is one short phrase in Class A of Part 1 of the General Permitted Development Order, and it is stricter in law than most people assume and looser in practice than most officers admit. The condition requires that the materials used in any exterior work are of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse, with conservatories excepted. There is no visibility qualification attached to it. A rear extension nobody can see from the street is caught by the same wording as a front one.

Similar appearance is also not the same as matching. That distinction is where the useful design work sits.

What similar in appearance means in practice

The test is visual and it is judged against the existing house rather than against an idea of what the street looks like. Two materials can be different products and still be of similar appearance. The same product can be manufactured in a way that reads entirely differently once laid.

Officers assess it at the distance the work will normally be seen from, which is why a slight difference in brick tone matters less than a difference in course height, and why a mortar colour mismatch can fail a wall built with the right brick.

Three practical readings follow from that. Course height, joint profile and mortar colour do more work than brick colour. Roof tile profile does more work than tile colour. And window frame proportion does more work than window material.

Brick matching and why blends fail

Start with the gauge. Bricks laid in a house built before the mid 1960s are frequently imperial, around 68 millimetres high, against the 65 millimetre metric standard. Over ten courses that difference accumulates to something the eye reads immediately at a junction, and no amount of colour matching corrects it. Imperial sizes are available and cost more, and on a visible junction they are worth the money.

Then the mortar. Joint width, colour and profile change the apparent colour of a wall more than the brick does, because on a typical wall a substantial share of the visible surface is mortar. A recessed joint reads dark. A weatherstruck joint throws a shadow. A flush joint in a pale mortar makes a wall look lighter than the brick sample suggested. Where the original wall is built in lime mortar, using cement changes both the appearance and the way the wall behaves.

Blends are the usual failure. A blended brick is manufactured to give variety in a new wall, and it produces a repeating pattern of tones that reads at ten metres as speckle rather than as the settled tone of a weathered elevation. A single-tone brick slightly darker than the original ages towards it. A blend does not.

Two further points on old walls. Weathering and soot mean the original brick as laid is not the colour you can see, so match to a cleaned area or to a sheltered part of the wall. And salvaged bricks vary in quality, so specify what proportion may be reclaimed and where they will be used.

Roof tile profile and colour

Profile first. A clay pantile, a double Roman, a plain tile and a modern interlocking concrete tile all sit differently in the roof plane and cast different shadows along the courses. Substituting an interlocking concrete tile for a clay plain tile roof fails the similar appearance test even where the colour is close, because the coursing and the shadow line are visibly different.

Slate divides on thickness and edge. Natural slate laid with a dressed edge and a random width reads differently from a uniform imported slate, and from a fibre cement slate with a manufactured edge. On a modest rear extension a good imported natural slate usually passes. On a front elevation in a conservation area it often does not.

Verge and eaves details matter more than the tile on small roofs. A dry verge system on an extension abutting a mortar-bedded verge on the host roof is the detail an officer notices first.

Windows and the frame depth question

Three dimensions decide whether a new window reads as part of the same house.

Reveal depth is the first. Traditional construction sets the frame back into the reveal, often by 75 to 100 millimetres, which produces a shadow line and depth. Modern construction sets it close to the outer face, which reads flat. Matching the reveal depth of the existing openings does more for the appearance than matching the frame colour.

Sightline width is the second. A slim traditional timber casement has a very different frame section from a standard plastic profile, and the difference shows most in the glazing bars and the meeting rail.

Glazing bar treatment is the third. A true divided light with real bars reads differently from an applied bar stuck to sealed glazing, and differently again from a spacer bar between panes. On a traditional house the applied bar is the compromise most conservation officers will discuss.

One modern constraint to design for: Approved Document F requires background ventilation, which in practice usually means trickle vents. Where those sit in the head of the frame they change the sightline, and the alternative positions worth exploring are over-frame vents or a through-wall route.

Contrast as a deliberate design choice

Contrast is a legitimate design approach and it is not available under permitted development. The Class A condition requires similar appearance, so a zinc-clad box on the back of a brick semi needs planning permission whatever its merits.

Where permission is being sought anyway, national and local design guidance is generally open to either approach: careful matching, or a clearly subservient contemporary addition that reads as a separate element. What loses is the middle ground, where the extension is nearly matching and visibly not. A contrast strategy needs three things to succeed in a submission: a clean junction with the host building, a subservient scale, and a material with a long-term appearance the officer can be shown on a completed building.

On a listed building the calculation changes again, because the test is the effect on special architectural or historic interest under the Planning (Listed Buildings and Conservation Areas) Act 1990, and a Heritage Statement identifying significance and assessing the effect is what carries the case. The wider permitted development test itself is set out in materials of similar appearance in permitted development.

Sample panels and conditions

Expect a condition. The usual wording requires samples or a sample panel to be submitted and approved before the relevant works begin, and it is a condition that must be discharged rather than noted.

Four things make the discharge quick:

1. Build a sample panel of at least a square metre on site, in the proposed brick, with the proposed mortar and the proposed joint profile, and leave it to dry before photographing it.
2. Photograph the panel next to the existing wall, in daylight, without flash.
3. Submit the product data sheets for brick, mortar, tile and window profile together rather than in instalments.
4. Where the wall is small, ask for the condition to allow approval of physical samples instead of a panel, and offer to bring the samples to the office.

Officers approve what they can see. A discharge application consisting of three product codes and no photograph is the reason these conditions take eight weeks.

Conservation area expectations

In a conservation area the local design guide usually names acceptable materials, and an Article 4 direction may have removed Class A altogether, which means an application is required for the extension in any event. The statutory duty is to pay attention to preserving or enhancing the character or appearance of the area, and material choice is one of the main ways that duty is applied to householder work.

Two habits help. Check the National Heritage List for England for the listed status of the host building and its neighbours before designing, because curtilage structures and attached neighbours change the answer. And set out the material choices in the Design and Access Statement with photographs of the existing fabric, since an officer reading a considered account of the existing wall is far more likely to accept a considered proposal for the new one. The same reasoning applies to side extensions on terraces, discussed in side return infill on a Victorian terrace.

Enter your postcode on PlanScope to see which reports your extension is likely to need.

This is general guidance, not legal advice. Permitted development conditions are technical and a lawful development certificate is the only formal way to confirm that works are lawful, so 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.