Dr Samuel Hughes is a Research Fellow at the University of Oxford and a Fellow of Create Streets.

In the seven decades since the introduction of the planning system, little development has been uncontroversial. The battles of recent months testify to how far we are from consensus on how to improve matters. But there is at least one way to create a substantial quantity of housing that should upset almost nobody, working in the best traditions of English building. Ironically, its name comes from the French: the mansard.

The mansard is a kind of double-pitch roof, traditionally lit with dormer windows. It is a little more costly to build than a single-pitch roof, but it effectively creates an extra floor, so the Georgians and Victorians added it when living space was valuable enough to justify the additional build cost. For centuries, this was a normal way in which English cities adapted to population growth, playing an important part in the great urban tradition of this country.

As a way of creating more housing, mansard extensions still have much to recommend them. They add more bedrooms in areas of high demand close to transport links, bringing new customers within walking distance of struggling high streets. Unlike building on green fields, they cost no countryside; unlike high-rise, they have negligible impact on the skyline. Because they involve adapting an existing building rather than creating new ones, they have a very low carbon footprint. In fact, adding mansards seems almost without downsides, provided of course that they are well-designed.

As we all know, much of Britain now suffers from an acute housing shortage. In virtually any southern English town or city, the floorspace value of a mansard extension would enormously exceed its build costs. Under such circumstances, the Georgians or Victorians would have swiftly added mansards to every suitable building. But in fact, almost none are added today, despite the fact that there are probably hundreds of thousands of terraced houses on which they would look natural. The cause is simple: it is that, with honourable exception, the planning system does not permit them. Amazingly, this tends to be true even for terraces with some older mansards, freezing in place an ugly ‘gap-tooth’ effect.

In my paper Living Tradition, just published with Create Streets, I argue that we should change this. On terraces where some houses have mansards already, the remaining houses should be allowed to add mansards too. On appropriate terraces on which no house currently has a mansard, the residents should be allowed to vote to create such a permission.

It might be feared that this will lead to a wave of hideous roof extensions, disfiguring historic neighbourhoods. But this can be avoided simply by requiring that new mansards follow strict design rules covering pitch, height, materials, parapets, party walls, chimneys, window form, and detailing. There are countless historical precedents for such rules, and there is nothing especially difficult about drafting or enforcing them. My report includes an outline of the rules we would need, drafted in consultation with many of Britain’s leading traditional and conservation architects: this should be refined with the help of the Georgian Group and the Victorian Society.

In recent years, some local authorities have begun to take a more flexible approach. The most celebrated case of this is Fitzroy Road in Primrose Hill, pictured below, which was formerly an extreme ‘gap-tooth’ terrace with just a single old mansard. The residents campaigned for permission to add more, which the council eventually granted on condition that every house add an identical mansard simultaneously. Remarkably enough, the residents accepted this condition, and entered into a joint contract to build the mansards in 2012. My proposals would offer similar opportunities to many streets similar to Fitzroy Road, but without requiring the resources to conduct such a long and highly coordinated planning battle.

Would this solve the housing shortage? The answer, of course, is no. But it would help. The UK has about 4.7 million homes from before 1918, disproportionately in areas feeling acute housing pressure like London. The minimum size of a flat in the UK is 398 square feet, probably a bit larger than the average mansard extension. So if just five per cent of pre-1918 buildings added mansards, we would add the floorspace equivalent of some 200,000 new minimum-sized flats, or about 130,000 average-sized ones (495 sq ft). For comparison, London added only 36,130 homes in 2018/19, the most recent year for which we have data. Many mansards would of course be extensions to houses rather than self-contained flats, but this helps to provide much-needed extra bedrooms for children or elderly relatives.

We have a Government that genuinely wants to address our national housing shortage by allowing more housing in areas of high demand, but which faces intense opposition to building high rise towers or in the countryside. This is also a Government that is keen to reduce pollution and to promote neighbourhoods where people live within walking distance of everyday needs. It is a Government that understands the value of design codes to ensure beautiful design. There are not many ways of creating homes that serve all of these aims simultaneously. This one must surely be of interest.