Structural Engineering in Clapham, SW4

Lambeth first designated the Clapham conservation area (CA01) in 1968, around the Common and its many eighteenth and nineteenth century buildings, and notes a great deal of mid to late nineteenth century housing beyond the Common's edges.

In Clapham Park, the appraisal says, most walls are London stock brick, and the early Victorian villas carry stucco dressings with render usually kept to the semi-basement so that it reads as a classical plinth; structural work in those houses often begins below the main floor.

Chimney breasts are the most common structural change. Lambeth's Design Guide accepts that removing an internal breast is common but asks for care over the structural impacts, and it discourages taking down external chimneys because of their effect on the roofscape.

Partial demolition of chimneys, turrets or parapets should be avoided where it would harm a building's design integrity, and where a loft or mansard raises a party wall, Lambeth asks for the party wall chimney stacks to rise eight brick courses above the roof.

On the roof, the same guide asks for raised party walls to be built up in stock brick following the slope, coped with bricks on edge, and for parapet gutters rather than gutters or fascias on the party wall, so nothing overhangs the neighbour.

Lambeth treats structure as a Building Regulations matter: structural works normally need separate Building Regulations approval, and its basement guidance says that structural stability is not a material planning consideration.

Where footings change, the neighbours are involved. The basement guidance asks whether deepening or stiffening the footings of a party wall could affect the structure on the other side, especially in a clay soil area, and notes that the Party Wall Act covers excavation within three metres of the party wall, or within six metres below a 45 degree line from the bottom of the neighbour's foundation.

The guidance also keeps the risk in proportion: in practice structural damage is rare and usually limited to minor cracking that can be readily repaired, and it suggests categorising the risk with the Burland Scale used by the Building Research Establishment and the Institution of Structural Engineers.

Lambeth calls extending downwards beneath old masonry houses, not designed to modern engineering standards, a challenging and potentially hazardous undertaking. Its basement policy allows one floor of homes below ground and none under a house's existing basement or semi-basement; it also asks for the construction management plan to be reviewed by a suitably qualified engineer.

On a listed house the requirement is firmer: structural surveys are needed for any basement work affecting or adjoining a listed building.

Before work on a shared wall the neighbour must be told, GOV.UK says, and if the two owners cannot agree, a surveyor is appointed, either jointly or one each. Lambeth adds that the Act is civil legislation, a private matter between neighbours in which the council cannot be involved.

Where underpinning is dug next to a neighbour, Lambeth's basement guidance says hand digging with a spade instead of an excavator may be an option, and it reminds owners that the stability of adjoining properties is a civil matter settled through party wall agreements.

Outside Clapham: Fitzjohns Avenue, NW3, where we built a basement and a rear extension for a house in north London.

We open up the floors and the chimney breast on each level before an engineer specifies a beam, so the design rests on what the walls actually carry rather than on assumptions.

Sources: Lambeth Council: Conservation area profiles, Clapham (CA01); Lambeth: Clapham Park and Northbourne Road conservation area character appraisal (November 2023); Lambeth Design Guide SPD Part 4: Building Alterations, Extensions and Retrofit; Lambeth Design Guide SPD Part 5: Basements; GOV.UK - Party walls and building work.