House type · c.1918–1939
The inter-war house
The best value in London, if the extension is designed against what the plot allows.
In short
The suburban expansion of the 1920s and 1930s produced most of the housing in Edgware, Stanmore, Harrow, Pinner, Northwood, Mill Hill and out into Hertfordshire. Cavity walls, concrete strip footings, generous plots and — critically — permitted development rights that still allow a great deal without a planning application.
What you actually find
Cavity walls, concrete footings, and space around the house
Two-leaf cavity brickwork with wall ties, a functioning damp-proof course, concrete strip foundations at a proper depth, a solid or suspended concrete ground floor, and a tiled roof on a purlin-and-strut arrangement that is usually straightforward to alter. Original windows were steel or timber casements and have almost always been replaced at least once.
The plot is the asset. Side space, a deep rear garden and often a garage or driveway mean an inter-war house can grow in more directions than a terrace can.
What usually works
Two-storey side and rear extensions, loft conversions, outbuildings
A wraparound or two-storey side and rear extension, a hip-to-gable loft conversion with a rear dormer, a garage conversion, and a detached garden building — a gym, studio or office — are the standard moves, and most of them can be substantially delivered under permitted development where the rights have not been removed by an Article 4 direction or a previous extension.
Because the structure is cavity-built on proper footings, the structural work is generally simpler and cheaper per square metre than the equivalent on a period terrace. That is where the value lies: more floor area for the same money.
Where the risk sits
Permitted development limits and cold bridges
Permitted development is a set of dimensional rules, not a general permission, and the commonest cause of trouble is a design that exceeds one of them — eaves height at the boundary, depth beyond the original rear wall, side width, or materials that are not of similar appearance. A lawful development certificate is inexpensive and removes the argument at sale.
Thermally, the weak points are the junctions: where a new extension meets the existing cavity, where a flat roof meets a wall, and around large glazed openings. Cold bridging at these points causes condensation and mould in otherwise well-built extensions. It is a detailing challenge, not an insulation-thickness one.
Finally, cavity wall insulation retrofitted in the 1980s or later is sometimes slumped or wet, and is worth inspecting by borescope before it is built over.
Typical scope
What these projects usually involve
A representative inter-war project is a two-storey side and rear extension with a loft conversion above, a reconfigured ground floor with a single large kitchen and dining space, a new roof covering, replacement windows, a full rewire and replumb, underfloor heating to the new ground floor, a new kitchen and three or four bathrooms, fitted joinery throughout, and a landscaped garden with a detached outbuilding.
Eternal is based in Edgware and works throughout this stock — from Edgware, Stanmore and Mill Hill through Harrow, Pinner and Northwood and into Watford, Bushey and Radlett.
Our work on this type
Projects delivered by Eternal on comparable buildings
Enquiries
Working on one of these?
Send the drawings, the survey or simply the address and we will tell you what we think the job actually involves.