OK. So, for my house I got a 5m wide by 4.5m deep rear extension no problem.
I am hoping to move into a similar house in the same area. I want to do a better, more adventurous extension, that might be in the region of 6.5m wide and 5m deep on one side, and 6.5m deep on the other (latest idea attached), or a diagonal wall (also attached and discussed in another thread). Both of these will connect to a garage, which is only used for storage. The garage will be converted to something, ideally gym/office, but if that is considered habitable space, maybe just a bigger gym. The house is detached and the neighbours won't be affected (other side of garage won't really see it, other than a bit of the roof from their upstairs bedrooms, and other neighour has their own single story extension that goes out about 4m, but there will be a gap of around 1m from the fence border (for my access down side of house). Overhead view also attached - my house in the middle, will replace the conservatory with extension. It would have a lowish roof. The left neighbour is to the west, so will have almost no effect on light - especially as shrubs on the border are currently as tall as it will be - it might possible block a little sunrise light in the height of summer (actually, scrap that, the garage roof will block that morning light anyway).