Opposite my children’s school there was a large (I would guess five bedroom) house that had been empty for a couple of years. Last year it was sold to a developer who by converting and extending it turned it into four flats. This year the developer turned his attention to the large garden. He chopped it in half and in one half built two new houses. In total one unoccupied dwelling has been turned into six occupied ones. No doubt this has all been very profitable for the developer, but I think most people would agree that this sort of development has benefits for the community too. This is in inner London where there is overall shortage of housing and very few sites with the potential for building new houses.
However this approach has its detractors as discussed in this post from May. The Conservative party don’t like classifying gardens as brownfield sites. And have recently announced their policy on back gardens. Part of their objection is the “thin end of the wedge argument” that they used as part of their objection to empty dwelling management orders. First the government confiscate unused houses, could unused back gardens be next?
"Across the country, there is growing concern about how John Prescott's planning rules are leading to leafy gardens being dug up and replaced with soulless and ugly blocks of flats," "Worse could be to come, with even harsher planning regulations on the way and the prospect of compulsory purchase of gardens for 'social' purposes. And if they're not going to build over your garden, Gordon Brown will tax it instead under his plans for a delayed, but still forthcoming, council tax revaluation." Say the Conservatives
Not so say the government :draft planning advice makes clear that while residential gardens are defined as brownfield land, "this does not necessarily mean that they are suitable for development".