Not to speak of landuse=farmyard, which could with a bit of squinting be considered farmland.
I just run a JOSM query "type:way closed nodes:50- (building=* | ( landuse=residential & areasize:-10000))" followed by a simplify, "building areasize:-3" and "delete". The most tedious part is downloading megabytes of nodes in tiny pieces because the server won't give you any more.

Hi Erick,
I was just wondering why you had *that* many changes on a few days and had a look at those buildings. That precision is a wee bit too much ;) Nodes are about once real-world centimeter apart and one building ends up having several hundred nodes. I bet that's rounder than the buildings are in reality. With one pixel on the sat image corresponding to something like 30cm, we just can't tell. Could you make them maybe like this one? way/568563066/history#map=19/-3.231659/36.561064
20 nodes feels reasonable for a building this size. Everything else just keeps you from downloading large areas and increases the load on the renderer for no visible difference.

cheers,
Matthias