Do you need planning permission for a SIPs extension?

There is no planning category for structural insulated panels. A council assesses size, position and appearance, and the wall could be brick, block, timber or panel for all the planning officer minds. What matters is whether your design fits inside permitted development, the national permission that lets householders extend without applying.

This is general guidance for houses. Flats and maisonettes have no permitted development rights for extensions, and nothing here is legal advice on your property.

The limits for a single-storey rear extension in England

Your extension is permitted development if it meets all of these.

“Original” means the house as first built, or as it stood on 1 July 1948. If a previous owner already extended, that depth counts against your allowance.

The larger sizes need a prior approval application. The council writes to your neighbours and, if any object, judges the effect on them. The fee is £249.

Side extensions

A side extension can be permitted development if it is single storey, no more than 4 m high and no wider than half the width of the original house. It cannot face a road. In conservation areas, National Parks and Areas of Outstanding Natural Beauty, side extensions always need permission.

The rule that catches SIPs: matching materials

A SIP wall has no outer face of its own. It is finished in whatever you choose: timber boards, fibre cement, zinc, render on a carrier board, or thin brick slips. The government’s technical guidance says the materials used in exterior work must be of a similar appearance to those of the existing house.

So a rendered extension on a rendered house passes. Brick slips chosen to match a brick house can pass, and so can a tiled pitched roof beside a tiled one. Charred larch or standing-seam zinc on a 1930s brick semi does not, however well it is detailed.

That is not a refusal. It simply moves you from permitted development to a householder planning application, where the council can and often does approve contrasting materials on a rear extension. The fee in England is £548 and the target for a decision is eight weeks. Flat roofs are treated more leniently by the guidance, since a flat roof seldom matches the main roof anyway.

On designated land the rule is stricter still: cladding any part of a house in timber, stone, render, plastic or tiles is itself outside permitted development.

Two storeys

Permitted development for a two-storey rear extension is narrow: 3 m deep at most, 7 m clear of the rear boundary, and effectively ruled out if you are within 2 m of a side boundary because of the 3 m eaves limit. Most need an application. The page on two-storey SIPs extensions lists every condition.

When permitted development has been taken away

Three situations remove the rights altogether, whatever you build with:

• an Article 4 direction, which councils use mainly in conservation areas;
• a condition on the original planning permission for the estate, common on developments from the 1990s onward;
• listed buildings, where listed building consent is needed for almost any alteration.

Your council’s planning register shows the first two. Search your address and read the decision notice for the estate.

Proving it: the lawful development certificate

You do not have to tell the council about permitted development work. Many people apply anyway for a lawful development certificate, which is the council’s written confirmation that no permission was needed. It costs £274 for a proposed extension, about half the householder fee, and it saves a great deal of correspondence when you come to sell.

For a SIPs build it is worth having. If there is any doubt about whether your cladding counts as “similar”, the certificate application settles it before panels are ordered.

Scotland, Wales and Northern Ireland

Scotland has its own permitted development classes with different measurements, and they are drawn more tightly for extensions near boundaries. Separately, almost every extension needs a building warrant from the council’s building standards service before work begins. Starting without one is an offence, and warrants take weeks, so apply early.

Wales follows a similar pattern to England but with its own order and its own limits. Northern Ireland has a separate system again. In each case read the national guidance for your address.

Planning is not Building Regulations

Whatever the planning position, every extension needs Building Regulations approval. That is where the panels do get examined: structure, fire resistance, insulation, ventilation and damp-proofing. The panel supplier’s engineer normally produces the calculations, and your designer or builder makes the application. What SIPs are explains the figures building control will look for.

What to do next

1. Measure what has already been added to the house since it was built.
2. Search the council’s planning register for Article 4 directions and estate conditions.
3. Decide the cladding. If it will not resemble the house, plan on a householder application and add two to three months.
4. Apply for a lawful development certificate if you are relying on permitted development.

Planning time is useful time. While an application runs, the panel drawings and engineering for your SIPs extension can be finished, so the factory is ready on the day consent arrives. The cost calculator will give you a budget in the meantime.