You're weighing up your options. A single storey rear extension gives you the kitchen-diner. Going to two storeys gives you the kitchen-diner and an extra bedroom. The build cost per square metre is lower for two storeys. The value uplift is bigger.
The trade-off? Planning permission is harder to get.
But how much harder? We have the data to answer that question properly.
The Numbers
PlanningLens tracks approval rates across every common extension type. Here's how they stack up, from the easiest to the hardest to get approved:
| Extension Type | Approval Rate | Decisions |
|---|---|---|
| Conservatory | 93.2% | 28,345 |
| Wraparound extension | 90.8% | 21,370 |
| Rear extension | 90.6% | 186,137 |
| Side extension | 89.8% | 102,146 |
| Front extension | 88.5% | 26,453 |
| Outbuilding | 87.9% | 46,069 |
| Hip-to-gable | 86.9% | 9,310 |
| Loft conversion | 86.0% | 28,968 |
| Dormer | 85.7% | 58,175 |
| Basement | 86.3% | 8,802 |
The pattern is clear. Projects that stay at ground level and out of sight — conservatories, rear extensions, wraparounds — have the highest approval rates. The further up you go, the more scrutiny you face.
Five percentage points might not sound dramatic. But on a base of 58,175 dormer decisions, that translates into thousands of additional refusals that wouldn't have happened at ground level.
Why Height Makes Things Harder
It's not arbitrary. Planning officers assess proposals against a consistent set of concerns, and almost all of them get worse when you add height.
Shadow and light. A single storey rear extension might shade a small patch of your neighbour's garden at certain times of day. A two storey extension will shade a much larger area, for longer. Planning officers use daylight tests (the 45-degree rule is common) and two storey proposals fail these more often.
Overlooking. Upper floor windows look down into neighbouring gardens and rooms. This is one of the most frequent grounds for refusing upper-storey additions. You can mitigate it with obscure glazing or high-level windows, but some councils won't accept that as sufficient.
Visual impact. A single storey rear extension is invisible from the street. A dormer or two storey extension changes the roofline or the rear elevation in a way that's visible to neighbours and, in some cases, from public spaces. This gives the council more material to work with if they want to refuse.
Scale and massing. Two storeys on a modest semi can look disproportionate. If the extension starts to dominate the original house, planning officers will push back on grounds of overdevelopment.
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The Council Factor
The five-point gap between rear extensions and dormers is a national average. In some councils, the gap barely exists — both types sail through. In others, particularly dense urban areas with small gardens and close neighbours, the gap can widen to 10 or even 15 points.
Councils where rear extensions are already tough (below 85%) tend to be even tougher on two storey proposals. If your council is at the stricter end of the spectrum, going to two storeys meaningfully increases your chances of refusal.
Conversely, in permissive councils (above 95% for rear extensions), two storey proposals still have strong odds. The planning culture in these areas is accommodating, and adding height doesn't trigger the same resistance.
When Two Storeys Makes Sense Anyway
None of this means you shouldn't go for two storeys. The additional space and value can justify the extra scrutiny — particularly if you check the landscape first.
Look at what's been approved nearby. If your neighbours have two storey rear extensions, you have precedent. If nobody on your street has gone above single storey, ask yourself why. It might be because nobody's tried. Or it might be because the planning environment doesn't support it.
Your architect should be able to tell you whether two storeys is realistic in your location. But an architect's view is based on experience and professional judgment. Our data is based on what's actually been decided — every application, every council, every ward.
Thinking about extending? Check the data first.
PlanningLens shows you approval rates for every extension type near your property — rear, side, dormer, loft, and more. See what gets approved and what gets refused in your area before you commit to a design.
Check Your Postcode — Free →The Smart Approach
If you're torn between single and two storeys, here's how to use the data. Start by checking your council's overall approval rates for both types. Then look at the ward level — is your ward more or less permissive than the council average? Then look at what's actually been built on nearby streets.
If the data supports two storeys, go for it — with confidence. If the data suggests it's borderline, consider whether the extra space is worth the possibility of a refusal that could cost you months and thousands in redesign fees. Sometimes a well-designed single storey extension that gets approved first time is the better outcome.
The best decision is an informed one. And informed starts with data.
See the full extension type breakdown: Extension Approval Rates — National Rankings
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