How Much Do House Extension Plans Cost in Sydenham?

Which Sydenham conservation area is my house in?

Sydenham has two that shape most extension questions, with others nearby.

Sydenham Hill, designated in 1976, mixes 19th century buildings with a 20th century estate (Lewisham, Sydenham Hill). The council describes three parts:

• Mount Gardens, a nearly rectangular site with unmade roads on three sides and dense trees that give it "an almost rural atmosphere", with detached houses of varied design.
• Mount Ash Road, a set piece of identical three-storey Victorian byelaw terraces, which has its own Article 4 direction.
• Lammas Green, built for the City of London Corporation in 1955 to 1957 and listed Grade II in 1998 in its own right.

The Victorian mansions along Sydenham Hill itself complete the area. Lewisham's SPD notes that the villas on Sydenham Hill and Eliot Bank survive from when Sydenham became fashionable after the Crystal Palace moved to Upper Norwood.

Sydenham Park, designated in 1973, has "a peaceful Gothic revival character" in yellow London stock brick with stucco dressings and slate roofs (Lewisham, Sydenham Park). Numbers 35 to 35d Sydenham Park have an Article 4 direction (Lewisham Article 4 directions).

Close by, Jews Walk (red brick Gothic revival pairs with diaper patterns, designated 1973) and Sydenham Thorpes (substantial red brick Edwardian houses on a grid of terraces, with its own Article 4 direction) follow similar rules.

Why does a sloping plot change a rear extension?

Because the kitchen floor and the garden are often a storey apart. On a plot that falls away from the house, a rear extension at kitchen level ends up raised above the lawn, and the first idea is usually a terrace on its roof or steps down from a raised deck. Lewisham's SPD says the council will not support external platforms, roof terraces, balconies and staircases to the side or rear above ground floor level where they are conspicuous and likely to cause overlooking, and that external stairs are unlikely to be supported in conservation areas (Lewisham SPD, paras 4.7.1 and 4.7.3).

So on a Sydenham slope we usually design one of three answers instead:

1. Step the extension down inside, so the new room meets the garden at its own level.
2. Keep the raised floor but no raised terrace, with stairs inside the footprint.
3. Dig a lower ground floor under the rear of the house, so the garden is reached from below.

The survey decides which is buildable. We record floor and garden levels on one datum, because a difference of a few hundred millimetres changes the height of the wall on the boundary and the answer to the neighbour's daylight. Our Forest Hill post explains Lewisham's 45 and 25 degree daylight tests on a slope.

How big can a lower ground floor be?

Smaller than people expect. Lewisham's SPD says basements should generally be no more than one storey below the original ground floor, and that a basement including its lightwells should not be wider than the original house nor extend more than 3 metres from the original rear wall or a quarter of the original garden, whichever is less (SPD, paras 4.8.8 and 4.8.9). It also asks that established trees and hedges are kept or replaced, which matters in the wooded gardens of Mount Gardens and along Sydenham Hill.

Digging into a hillside brings retaining walls, drainage and structure, and the neighbours' foundations, so a lower ground floor also needs a structural engineer's input and, usually, party wall notices to the neighbours. Both are specialists' work outside the planning fee, confirmed at the quote stage.

Can I add a side extension to a Sydenham villa?

It depends on whether the house is one of a pair. The SPD says a single-storey side extension should usually be no wider than half the original frontage and set back at least 150 mm from the front; a two-storey one should be set in at least 1 metre from the boundary and back at least 1 metre from the front (SPD, paras 4.4.4 and 4.4.6). It also allows that where the land slopes, a side extension may be taller than its neighbour if no harm is shown.

In conservation areas there is a further test. Where a building is part of a symmetrical pair or a stylistically cohesive group, a side addition is unlikely to be acceptable, because it unbalances the pair. The gaps between houses, the SPD says, intentionally allow views of foliage in rear gardens. That describes the semi-detached Gothic villas of Jews Walk and Sydenham Park closely, so for those houses we test a rear or lower ground option before a side one. Our Lewisham house extensions page sets out the rear extension limits.

What if my house is on Lammas Green or Mount Ash Road?

Lammas Green is listed Grade II, so any alteration that affects its character needs listed building consent as well as, or instead of, planning permission. On Mount Ash Road, the Article 4 direction means visible external changes need an application; Lewisham says its directions generally reach windows, doors, roofs, porches, front boundaries and painting on houses in single-family use. If your home is a flat in a converted villa, it has no permitted development rights, so every external change is an application there too. We confirm the drawings a listed building needs at the quote stage.

What does the £795 include?

One fixed fee for one extension, staged on our pricing page and explained on our house extensions page:

1. The laser measured survey, with levels front and back tied to a datum.
2. Existing plans, elevations and sections, cut through the slope.
3. Proposed drawings revised with you until the design is right.
4. Forms, location and block plans, and submission to Lewisham.
5. Replies to the case officer and any revisions they ask for.

Where Lewisham wants a heritage statement for a conservation area, ours is from £565; see our Lewisham heritage statement page.

How long will Lewisham take?

Lewisham decided 393 of 413 householder applications within 8 weeks in the year to June 2026, prior approvals excluded (MHCLG live table PS2). Conservation area cases are sent to the conservation team, so drawings that show the slope, the side gaps and the original details clearly are the surest way to avoid a request for more information. For a loft rather than an extension, see our Catford loft post; the Lewisham borough page has the council's full checklist.

Frequently asked questions

Can I build a raised terrace on my Sydenham rear extension? Usually not. Lewisham's SPD says it will not support platforms and stairs above ground floor at the side or rear where they are conspicuous and cause overlooking, so we design the level change inside the extension.

Is a lower ground floor extension possible on a Sydenham slope? Often, within limits: one storey down, no wider than the house, and no more than 3 metres from the original rear wall or a quarter of the garden, whichever is less.

Can I add a side extension to a semi-detached house in Jews Walk or Sydenham Park? It is difficult. In conservation areas Lewisham treats a side addition to one of a symmetrical pair as unlikely to be acceptable, so a rear or lower ground option is usually the stronger application.

Does Sydenham Hill have Article 4 directions? Mount Ash Road has one; numbers 35 to 35d Sydenham Park have another. Check your exact address with Lewisham, because directions apply to named houses.