There’s a specific order to answering this question in Esher, and cost sits fourth. Green Belt status sits first, because it decides whether a double storey extension is a design question or a policy argument. Get the sequence wrong and you pay for drawings that were never going to work.
Four diagnostic questions, in order. Each one determines whether you continue to the next.
Question One: Is Your Plot in the Green Belt?
Elmbridge contains a substantial amount of Green Belt land, and it runs through and around Esher rather than sitting neatly at the edges. Two houses on the same road can fall on different sides of the boundary.
Check this on the council’s mapping for your specific address, not your street.
If no: skip to question three. Standard permitted development and planning rules apply and your position is much simpler.
If yes: continue. Everything below governs what you can build, and it’s a different framework to the one most extension advice describes.
Question Two: How Big Was the Original House?
This is the question that ends more Esher schemes than any other, and almost nobody asks it first.
Green Belt policy in the NPPF treats extensions as inappropriate development unless they don’t result in disproportionate additions over and above the size of the original building. The framework defines the original building as it existed on 1 July 1948, or as originally constructed if built later.
Not as it stands today. As it stood in 1948.
So if two previous owners have each added a rear extension and a garage, your remaining allowance is measured against what was there originally, minus everything added since. Your predecessors may have spent your budget.
How to establish it: pull the property’s planning history through the Elmbridge portal. Historic aerial imagery and Land Registry documents help fill gaps. On older houses this is genuine research, and it’s the cheapest research you’ll do on the whole project.
If the original house was small and the additions substantial: a double storey extension is unlikely to work as proposed. Continue to question four anyway, because alternatives exist.
If there’s headroom: continue to question three.
Question Three: What Does Disproportionate Actually Mean Here?
Here’s the part that frustrates people. The NPPF doesn’t define disproportionate. It’s left as a matter of judgement for each local planning authority.
Different councils apply it differently. Some assess floorspace, some external volume, some footprint. Published guidance across authorities commonly sits somewhere between a 30 and 50 percent increase over the original, but the figure and the measurement basis both vary, and some authorities publish no fixed percentage at all.
That means you need Elmbridge’s own adopted policy and supporting guidance rather than a general rule of thumb from a national article. Check what measurement basis the council uses and whether detached structures in the curtilage count.
Two practical points that apply broadly.
Cumulative effect matters. Councils are alert to schemes that arrive in stages, each modest, adding up to something clearly disproportionate. A second application on a house that’s already been extended gets read against the whole history.
Design language matters too. Guidance across Green Belt authorities tends to expect extensions to be compact and to fit in with rather than take over the original building. A double storey addition that reads as a new house attached to an old one struggles even where the arithmetic works.
Question Four: Is Permitted Development Available at All?
Worth checking, because it changes the route entirely. For most Esher double storey schemes the answer is no, for reasons unrelated to Green Belt.
Under GPDO 2015 Class A, a rear enlargement of more than one storey must not extend beyond the original rear wall by more than 3 metres, and no part can be within 7 metres of any boundary opposite that rear wall.
The 7 metre rule ends most schemes on its own. And where any part of the extension sits within 2 metres of a boundary, eaves are capped at 3 metres, which is geometrically incompatible with a usable first floor.
Two storey side and front extensions aren’t permitted development at all.
So the realistic position for most Esher properties is a full planning application. That isn’t a setback. It means the design can respond to the plot rather than to a table of caps that was never drawn with Green Belt in mind.
Where That Leaves You
| Your position | Realistic route |
| Green Belt, generous original house, modest additions to date | Full application, design kept compact and subordinate |
| Green Belt, small original house, heavily extended already | Reconsider scope, or look at replacement dwelling |
| Green Belt, tight plot near boundaries | Single storey plus loft usually goes further than double storey |
| Not Green Belt, garden over 7 metres deep, clear of boundaries | Permitted development possibly available |
| Not Green Belt, urban plot | Full application, standard amenity assessment |
The Alternative Most People Don’t Consider
Where the disproportionate additions test blocks a double storey extension, two other routes are worth testing before abandoning the idea.
Loft conversion plus single storey rear. Class B allows additional roof space of 40 cubic metres for an attached house and 50 for a semi detached or detached one, where rights survive. Roof space and a modest ground floor addition together often deliver comparable accommodation with a much smaller effect on the volume calculation.
Replacement dwelling. Green Belt policy treats the replacement of a building as an exception provided the new building is in the same use and not materially larger than the one it replaces. Where an existing house is unremarkable and already heavily extended, replacement is sometimes a cleaner planning argument than extending further, because the baseline is the existing building rather than the 1948 original.
That distinction is significant and frequently overlooked. It’s one of the first things worth raising with architects esher homeowners consult, because it can reopen a project that looked closed.
What to Do This Week
Establish Green Belt status on your address. Pull the planning history and work out what the original house was. Read Elmbridge’s adopted Green Belt policy and note which measurement basis it uses. Measure your rear garden depth.
Four tasks, all free, all available online. They tell you whether you’re designing an extension or making a case, and those are very different projects with very different budgets.