One of the most common misunderstandings we hear from clients planning a double storey extension is that the rules are the same everywhere. They aren’t. A client reads a planning guide, assumes it applies nationally, then finds out their specific council has different setback distances or design requirements — and the extension they had in mind doesn’t fit what’s actually allowed on their plot.
Why councils treat double storey extensions differently
Most councils want a double storey extension to look like an addition to the house, not a replacement for it — the extension should read as clearly secondary to the original building, not compete with it. That’s the underlying design principle behind most of the specific rules you’ll come across, even though the exact numbers differ from borough to borough.
The typical pattern looks something like this:
A setback from the boundary — commonly around one metre, particularly on the side wall
The first floor recessed from the ground floor, so the upper storey doesn’t sit flush with the property line
A lower ridge height than the main roof, so the extension doesn’t compete with the original roofline
Most councils also cap the width of a double storey extension relative to the original house — commonly no more than half, or up to two-thirds, of the property’s width.
Some councils take a different approach and allow an integrated design flush to the boundary. Hounslow is a good example: rather than the usual one-metre set-in from the boundary, Hounslow will typically allow you to build right up to the boundary line, provided the extension doesn’t exceed two-thirds of the property’s width. That’s a genuinely different approach from most other boroughs, and it’s exactly the kind of thing that catches people out if they’ve read guidance written for a different council.
Why you can’t assume your neighbour’s rules apply to you
Every council publishes its own residential design guidance and planning policies, and these documents aren’t always easy to find or interpret if you don’t do this regularly. That’s exactly why we get asked this question so often: a client’s friend, colleague, or neighbour built a double storey extension in a different borough with different rules, and the client assumes the same numbers apply to their own property.
They don’t. Setback distances, width caps, subordination requirements, and ridge height rules are set locally, which means the honest answer to “how big can my double storey extension be” always starts with “it depends on your council.” A one-metre boundary set-in is typical — but Hounslow’s build-to-boundary allowance shows just how much that can vary.
How to find out what applies to your property
If you’re not sure what your council’s specific requirements are, you don’t need to dig through the planning portal yourself. Send us your property address by email or WhatsApp and we can tell you what the rules are for your specific plot — because we know where to look and what to look for.
Read more: Building on Existing Extensions — The Structural Assessment You Need & Wraparound Extension: Why You Can’t Do It Under Permitted Development
