Rear extension: Permitted Development or planning application?

When homeowners ask us about a rear extension, the first question is rarely about design. It is about permission: will this need a planning application, or can it go ahead under a lighter route? The honest answer is that it depends on the property, and we would rather explain how the routes differ than offer a quick reassurance that may not hold for your house.

The three routes, in plain terms

In broad terms, a single storey rear extension in England tends to follow one of three paths. Which one applies depends on the size and height of what you propose, the type of property, and where it sits.

Permitted Development

Many modest extensions may fall within Permitted Development, which means the work can proceed without a full planning application, provided it stays within national limits on depth, height and proximity to boundaries. Those limits are detailed and are interpreted locally, so we treat any early assessment as a starting point rather than a conclusion.

Prior Approval for a larger home extension

Where a single storey rear extension is deeper than the standard Permitted Development limit, a Prior Approval route may be available. This is not a full planning application, but the council does consult neighbours and can decide whether the proposal is acceptable. As a rule of thumb, in our experience a decision on a Prior Approval takes around six to eight weeks.

A full planning application

Some projects need a full application: a double storey extension, a design that wraps around the side of the house, or anything that does not fit the lighter routes. A standard application typically takes eight to thirteen weeks to reach a decision, so it is worth allowing for that before you fix a start date.

What can change the answer

Even a modest design can move from one route to another once the details of the property are considered. These are the points we check first:

• Whether the house is a flat or maisonette, which do not share the rights of a house
• Whether the property is listed or sits within a conservation area
• Whether an Article 4 direction removes rights that would otherwise apply
• Whether earlier extensions or a converted loft have already used up part of the allowance
• Whether a condition on the original planning permission restricts further building
• How close the new walls would be to a boundary, and how tall they would stand

None of these is unusual in Surrey and South West London, but any one of them can change the route. Our guide to planning permission for home extensions sets out the considerations in more detail.

Building Regulations are a separate matter

It is easy to assume that a project which avoids a planning application is free of oversight. It is not. Building Regulations approval is needed whichever planning route applies, and it covers structure, drainage, insulation, fire safety and ventilation. Where the extension opens up an existing wall, the structural calculations for the beam need to be in place before work reaches that stage.

How the route affects your programme

The planning route shapes the calendar more than most people expect. A project that needs a decision from the council has a waiting period before anything starts on site, and that period is outside anyone's control. Once approvals are in place, a single storey rear extension typically takes ten to fourteen weeks from foundation to completion, while a double storey or wrap around extension is more commonly sixteen to twenty four weeks.

For that reason we like to settle the planning route early, ideally before detailed design is finished. A design drawn to suit the right route is cheaper to change on paper than on site.

Questions worth asking before you commit

If you are weighing up an extension, these questions help to frame the first conversation with any builder:

• Which planning route do you expect this design to follow, and why?
• Who prepares and submits any application or Prior Approval notice?
• What happens to the programme if the council asks for changes?
• When are the structural calculations produced, and by whom?

A builder who can answer these clearly before work begins is usually a builder who has done it many times. You can see how we approach the work on our house extensions service page, and our cost guide explains what shapes the overall figure without quoting a number that would not apply to your home.

If you are buying a house to extend

The planning question becomes more pressing when you have not yet bought the property. Rights that appear to be available may already have been used by a previous owner, and a seller's earlier extension can quietly reduce what is left for you. This is where our Pre-Purchase Consultation is most useful, because it gives you a builder's view of what is realistic before you commit to the purchase.

A sensible next step

We confirm the likely route for your specific property as part of every Cost Proposal, so you know where you stand before any design fee is spent. If you are planning a rear extension in Surrey or South West London, we would be glad to provide a free Cost Proposal.