Planning authorities in the UK generally require that a proportion of housing on developments over a certain size is set aside as 'affordable' or else that a commuted sum is paid to the planning authority in order that they may provide affordable housing elsewhere. The definition of affordable here doesn't matter except to note that it is housing at below market prices (either for rent or sale). There is little political objection to this principle other than its degree and in the impact it might have on the viability of development.
The objectives of such policies are firstly to ensure that there is an adequate stock of affordable and social housing so as to meet need, and secondly that communities are 'integrated' (meaning that we don't repeat the errors of the past by building isolated or separate social housing 'ghettoes'). And the use of planning obligations seems to make sense as it ensures delivery.
However, these excellent policies may contain a problem by reducing supply of new housing across all tenures and housing types. In the USA these policies are called 'Inclusionary Zoning' (or IZ) - so named to set them as different from exclusionary zoning that it used to prevent the development of low cost housing - and a couple of US economists, Tom Means and Edward Stringham, have shown how these policies might not be the solution they're cracked up to be:
“Between 1980 and 1990,” they write, “cities imposing below-market housing mandates end up with 9 percent higher prices and 8 percent fewer homes overall. Between 1990 and 2000 cities imposing below market housing mandates end up with 20 percent higher prices and 7 percent fewer homes overall.”
Put simply the imposition of requirements to provide affordable housing on a development site (or else pay a commuted sum) seem to result in less housing being built and, unsurprisingly, house prices being higher. Now it may be that the UK's housing market will behave differently - the problem is that most research into affordable housing here is either funded by affordable housing providers or else focuses on affordable housing supply rather than supply in general.
Like most policies around housing the real issue is hidden in a thicket of detailed studies. We need below-market prices for housing because housing is expensive. Housing is expensive because the land on which the houses are built is expensive. The high land price is a consequence of there being a limited supply of said land. And the limited supply is (to a considerable extent) a matter of policy choice:
The reason, after all, that housing is expensive in places like New York, San Francisco, DC, and other IZ-friendly (not to mention rent-controlled) locales is that zoning regulations prevent supply from keeping pace with population growth. In response, public officials have pursued demand-side solutions to this fundamentally supply-side problem, through a massive subsidy apparatus that includes rent-control, public housing, Section 8, tax credits, and now inclusionary zoning. Combined, these measures are band aids for a mortal wound, since they ignore the underlying cause of why hot urban housing markets remain expensive. And sometimes, by discouraging construction, these measures worsen the problem.
Translated into English this tells us that, where you have pressured housing markets (something that is true in London but not in Bradford), the answer lies in more supply of land and allowing higher densities rather than regulating or subsidising housing. Almost everyone who looks at the problem realises pretty early on that the barrier to our supply problem in housing isn't finance, isn't government funding and isn't the market. The problem is the planning system.