To be honest, when I started doing this I was following the approach used by blackadder (https://www.openstreetmap.org/user/blackadder) in Sutton Coldfield, which had been praised by other users. The effective output is to see a land parcel, similar to that which is displayed on the highest detail of OS MasterMap, by utilising local knowledge and OSM Cadastral information. You are right in that the building sits 'on' the garden, but the garden itself is utilised more as a 'delineating feature' for the association of the land to the property. Clearly, there are going to be no pansies underneath the properties.

I hear what you are saying, but you are also adding another level of complexity and detail, and lest us not forget, we cannot know what the composition of every boundary is anyway, so its never going to be a perfect exercise.