How Much Do House Extension Plans Cost in Chislehurst?

Chislehurst is different from the rest of Bromley because so much of it is protected. Before anyone discusses square metres, three questions decide the route: is the house in the conservation area, is the plot in the Green Belt, and does an Article 4 direction cover the estate? Many Chislehurst plots answer yes to at least one.

How big is the Chislehurst conservation area?

Large. It was first designated in February 1972, with Chislehurst East and Chislehurst West added in March 1982 and a small boundary change in 1999. At roughly 3 km by 2 km, it is the largest of Bromley's 47 conservation areas. It runs from Chislehurst station in the south to the borough boundary in the north, mostly residential in the west and rural and agricultural in the east (HCUK Group heritage statement, 2021; Bromley Council). Supplementary planning guidance for the area was produced in 1999.

That guidance describes the large houses that give Chislehurst its character: English vernacular and Arts and Crafts homes on spacious plots within established woodland, sometimes individually designed, often built as speculative developments. A 2014 Bromley committee report on a house in Southill Road quotes its paragraph 4.24: extensions should reflect the forms, materials, textures and finishes of the host building and the design philosophy behind its style, and should not be so large as to dominate it (Bromley report, application 14/03055).

So the drawings do more than show a bigger kitchen. They record the house's materials and details, show the new work in the same language or a clearly deliberate contrast, and come with a heritage statement explaining why the change preserves the area. Ours is from £565; our heritage statement post explains what it includes.

What can I build without planning permission in the conservation area?

Less than elsewhere. In a conservation area, an extension beyond a side wall, a rear extension of more than one storey, any roof extension and cladding are not permitted development (MHCLG technical guidance). The larger home extension prior approval route is not available on that land either. A modest single-storey rear extension within the normal depths may still be permitted development, so we check that first.

For most Chislehurst owners inside the boundary, though, a side extension or anything upstairs means a full application judged against Local Plan Policy 41, which asks new work to respect the layout, scale, form and materials of the buildings around it and to use high-quality materials (Bromley Local Plan, chapter 5).

Does the 1 metre side space rule apply on larger Chislehurst plots?

As a minimum. Policy 8 asks for at least 1 metre between a two-storey extension and the side boundary, for its full height and length, and a more generous gap where the area already has wider spacing (Bromley Local Plan, chapter 2). On Chislehurst's wide plots, 1 metre is often not enough. In a 2016 report on a house in Sandy Ridge, the officer accepted 1 metre partly because the house was outside the Chislehurst conservation area (Bromley report, application 16/02161). Inside it, expect the officer to ask for the gap that the street already shows.

Our survey therefore measures both side gaps and the gaps between the neighbouring houses, so the drawings show the rhythm of the street and not just your plot.

What if my house is in the Green Belt?

Then the limit is arithmetic. Bromley Local Plan Policy 51 allows an extension to a house in the Green Belt or on Metropolitan Open Land only if the net increase in floor area over the original house is no more than 10 per cent, measured externally. The extension must also not harm the open or rural character of the area, or change the form and bulk of the original house significantly. Extensions to converted or replacement dwellings are not normally permitted (Bromley Local Plan, Policy 51).

The word "original" does the work. Earlier extensions, conservatories and some outbuildings count against the 10 per cent, so a house extended in the 1980s may have little or none left. The Green Belt is tested at appeal here too. In August 2024 an inspector dismissed an appeal for new homes at Home Farm, Kemnal Road, weighing harm to the Green Belt and to the conservation area together (appeal decision).

For a Green Belt plot we work out the original floor area from planning history and the survey before any design, and give you the remaining allowance in square metres.

Which Chislehurst estates have Article 4 directions?

Two appear on Bromley's list (Bromley Article 4 direction areas). The Marlings Park Estate, an Area of Special Residential Character, has a direction covering Part 1 Class AA, the right to add storeys to a house. There is also a direction for Kemnal Road, Chislehurst; we read its text for the rights it removes before advising on any house there. Do not confuse either with the Chislehurst Road conservation area direction, which covers a road in Petts Wood (see our Orpington garage post).

What does Bromley need with the application?

The same checklist as the rest of the borough: a biodiversity net gain exemption statement, roof plans for any first-floor, two-storey or roof work, and photographs taken within 30 days of submission. In Chislehurst add the heritage statement, and a tree survey if building work comes near significant trees. That survey is a specialist's job, and we tell you at the quote stage whether one is needed. Our Beckenham two-storey extension post covers the checklist and the side space rule in more detail, and our Bromley house extensions page and Bromley page cover the rest of the borough.

Frequently asked questions

Is all of Chislehurst in the conservation area? No. It is large but has a boundary; Sandy Ridge, for example, was outside it in 2016. We check your address against Bromley's map before advising.

How is the Green Belt 10 per cent measured? Externally, against the floor area of the original house. Earlier extensions count, which is why we start from the planning history.

Can I add a storey in Marlings Park? Not under permitted development. The Article 4 direction withdraws that right, so an upward extension needs a planning application.

Do I need a heritage statement? In the conservation area, yes, as part of a sound application. Ours is a separate fixed fee, confirmed at the quote stage.