Hi all, posting this as a bit of a frustrated post, but also as a "well if they did maybe this can help you" as I know a few people in our part of London that are struggling with room for au pairs/family and the like and the law should apply to us all equally!
Our neighbour (owner of the bottom floor of a two-apartment terrace - not the full freeholder) decided to add "another bedroom" by building a garden shed in the backyard. Without permission, a concrete slab was poured, builders in and the shed erected (knocked down our other neighbours fence and accessed through their backyard!!). Its nice enough. It has electricity, insulation, locks etc - everything but running water. Inside its been fitted out as a double bedroom with wardrobes and a desk and its rented out on AirBnB and sparerooms.com for over a year now. Its now been included as a "studio/bedroom" on floorplans for a real-estate agent listing for the property.
The Council was notified by the top-floor owner of the shed construction and its use as a bedroom (and as a guest house) and they told the ground floor owner to apply for retrospective planning permission to keep the shed.
Despite objections from neighbours on three sides, as well as the top-floor property owner AND there already being permission for a side-and-back extension on the property (which if completed would meant the backyard is completely built over as the shed takes the remaining space), the planning permission/right to retain the shed was granted. Its use as a bedroom has been considered ‘incidental or ancillary use’ to the property, which although specifically says that should not include bedroom space, the Council says that this is reviewed on a "case-by-case basis" and provided an inspectors report to support their decision/ruling.
So - if you want an extra bedroom on your property and don't mind upsetting all the neighbours, it seems that you can go ahead and build it, get retrospective permission to keep it, then use it with the approval of the council. As one of the neighbours impacted by this, I don't recommend it as a nice thing to just go ahead and build it in the way my neighbour has, but perhaps in this example there is some information for someone who is struggling with space issues and have amenable neighbours or a whole property and it might help. I know of another neighbour who wanted to house their daughter and her fiancé in a similar conversion while they saved for a deposit but were told it wasn't possible, although they owned the entire house. They didn't have the report that the Council used to support 'ancillary use', maybe with these examples their case would've been considered differently? Anyways, putting this out there if anyone can find it of use to them. Message me if you'd like any specifics sent over.
Please be nice to your neighbours! We're all just trying to live here together!