See this useful attached article from the splendid Estates Gazette Practice & Law section last month.
In short, the description of the planning permission granted may purport to limit what’s built, but unless the permission ALSO has a condition to the same effect, the “limitation” in the permission description may not be enforceable.
So, if you have a permission described as for “28 dwellings, of which 10 are to be affordable”, but want to increase the affordable to (say) 15 out of the 28, and the planners are saying you can’t (yes, unbelievably, it happens in some LAs!), send them a copy of this article……..Planning limitations vs conditions Jan2015