Westminster architects | Residential architect projects

Conservation areas in Westminster

Since the introduction of conservation area legislation in 1967, the City Council has designated 53 conservation areas which now cover three quarters of the City and include most of the central area.

The character and appearance of these areas vary considerably but all are of special architectural or historic interest. Some, such as the Strand, Trafalgar Square, Whitehall and Westminster Abbey and Parliament Square, have seen development and change from Anglo Saxon times and they contain some of the most historic streets and buildings in London. Others have much more recent origins, such as the Lillington Gardens, Hallfield and Churchill Gardens Estates, where the land was redeveloped for housing after the Second World War.

Westminster residential architect and planning advice – Extensions

Extensions to buildings in conservation areas should preserve or enhance the character or appearance of the area. They should in general be confined to the rear or least important facades and should not upset the scale or proportions of the building or adversely affect the character, appearance or setting of neighbouring buildings.

Extensions to the front or street facades of buildings will normally be unacceptable. This is because it is generally the street facades which make the greatest contribution to the character and appearance of a conservation area. Extensions which involve infilling of light wells at basement level at the front will not normally be acceptable, as these light wells are an important characteristic of many conservation areas, and should be retained.

More change is often possible at the rear, or occasionally the side, of buildings, without affecting adversely the character or appearance of conservation areas. At the rear, buildings have often been subject to many alterations over the years and the uniformity that often exists on the street frontage of a terrace may not be found at the rear. Rear areas are also less public, in that they cannot normally be seen from the street and are only overlooked by adjacent properties.

However, the design of rear extensions and alterations must respect the architectural character of the existing building, the terrace of which it is part (if it is within a terrace) and the conservation area as a whole. Extensions should respect the scale and proportions of the existing building and must be architecturally subordinate to the main building.

Normally the extension should terminate at the penultimate storey level, or lower, thereby leaving the existing parapet line unaltered. The depth of the extension needs to respect any existing addition on the building or the terrace of which it is part. Extensions, which cover the whole of the plot, are normally unacceptable. A significant area of garden or amenity space should normally be retained at the rear. This is particularly important in residential areas but may be less so in some commercial areas, such as Soho. If the building has an L-shaped plan form at the rear then this should normally be retained i.e. the lightwell should not be infilled, except for a glazed, conservatory type extension at ground floor or basement level. Generally, full width extensions are not acceptable, except in certain circumstances at basement level.

Westminster residential architect and planning advice – The design of roof alterations and extensions

Some roofs can be adapted to provide accommodation without major alterations and extensions. Tall pitched roofs may only need internal changes and/or the addition of a rooflight or dormer window. However, other roofs, such as the ‘M’ roof may need substantial alteration or demolition and replacement of the existing roof structure to provide additional accommodation. The form of roof extension that is most common in the City is the mansard roof.

Westminster residential architect and planning advice – The mansard roof 

The principal aim of this guidance is to assist in the design of mansard roofs which will be appropriate extensions on a large number of buildings in the City, where the principle of a roof extension is considered acceptable (see policies DES 6 iii and DES 8 vii).

The guidance is general because it can not deal with every circumstance that exists across the City but it sets out the fundamental rules which need to be followed. It is important to produce a roof form that is appropriate to the building and, if it is a terraced property, to the terrace of which it is part. In some cases it may be appropriate to deviate slightly from the guidance in order to preserve the uniformity that exists in a terrace, so long as the existing pattern of roof extensions is one which is appropriate and one which should be perpetuated.

The mansard roof is not only appropriate for many 18th and 19th century houses but also some classically designed commercial buildings of the 19th and 20th century. It may be inappropriate for many other building types. It is always important to consider carefully the architectural style and character of a building before designing any extension or alteration to it.

Westminster residential architect and planning advice – General rules

1. The principle slope should be pitched no greater than 70 degrees.
2. The upper slope should not normally be greater than 30 degrees.
3. The floor-to-ceiling height should be kept to a minimum. The normal maximum will be 2.3m (in domestic buildings).
4. The intermediate ridge, between the principal and upper slopes (the knee), should be kept as low as possible.
5. The roof should be set back behind a parapet gutter at the front and the rear.
6. The party wall slope should start behind the back line of the parapet coping.
7. The party wall profile should be parallel to the roof slopes, at the minimum upstand permitted under the Building Regulations (1991) of 375 mm.
8. Where a roof is extended the chimney stacks should be raised to retain the same height relationship with the roof.
9. Chimney stacks containing active flues must be raised to at least 1000 mm above the level of the adjoining roof covering to accord with Building Regulations (1991).

There are two types of mansard roof:-
a. The flat topped mansard
b. The double pitched mansard
In many cases the double pitched mansard may be the most appropriate. This is especially so on listed buildings because this form has been used from the early 18th century onwards and because it has a number of advantages, such as providing space for water tanks and other plant, within the roof space.