You can extend in Chislehurst, but you'll need permission for most of it. There are over a hundred listed buildings. A dozen conservation areas cover much of the ground. Hundreds of planning applications go in every year. It can be done, but expect the process to take a while.
Chislehurst has 104 listed buildings. One is grade I, five are grade II* and 98 are grade II. Listing covers the whole building, inside and out, so changing a listed house needs listed building consent from Bromley council on top of any planning permission. That applies to windows, walls and fireplaces as well as extensions. Grade II is the commonest grade and the least strict, but it's still strict enough that owners of a grade II house should expect to apply for most changes.
Twelve conservation areas fall within the area's boundary, among them Chislehurst itself, Bickley Park, Christ Church, Garden Road and Longlands Road. Being in one brings tighter control over demolition, over trees and over changes that can be seen from the street. Chislehurst has been designating conservation areas since 1971, after wartime bombing destroyed many of its trees and Victorian buildings and the post-war building boom threatened more. Much of the area is protected because a lot of it was nearly lost.
There is one scheduled monument within the area's boundary. That is the strictest level of protection there is, and it only matters for land close to the monument.
People still extend their homes in large numbers. Bromley's planning register shows 1,189 applications from Chislehurst between April 2024 and June 2026, 539 of them in the latest twelve months. Those are applications, not approvals, and the register doesn't record outcomes consistently enough to give a success rate. With that many applications, the council will almost certainly have seen something like yours before. That doesn't mean it will approve it.
The British Geological Survey puts every postcode in Chislehurst in class 3 for shrink-swell, which is the tendency of clay soils to swell when wet and shrink when dry. For an extension, that usually means deeper foundations than a builder might quote without checking, especially near large trees, and getting a structural engineer to assess the ground early. It affects the design and the budget, but it shouldn't stop a project.
Six listed buildings in the area are on Historic England's Heritage at Risk register. That means they are in poor condition, their repair may qualify for grant funding, and any work on them gets close scrutiny. They are a burden for their owners and say nothing about the ordinary house next door.
Permitted development rights let many houses add a modest rear extension or loft conversion without a full application. In a conservation area those rights are narrower, and on a listed building almost nothing counts as permitted. Bromley council's online maps show whether a particular house is listed or inside a conservation area, and that tells you which set of rules the extension will face.
Chislehurst is restrictive about extensions. The same protections that keep the place looking the way it does are the ones that will say no to a large rear extension.