Lots of good thoughts on this thread, agree with all of the above. This happened to a former colleague but we are no longer in touch so unsure what happened.
Agree it's likely to go through, with conditions. Local authorities are legally obliged to increase dwellings and they like it when they get a freebie like this (no cost to them and no increase in development footprint).
You might be able to identify the owners of the tenanted flats by paying £3 to access the land registry and see the registered owner. It only gives you a name but that may be enough to find them on linkedin. Bit of a gamble but it's only £3 per apartment.
You need to badger the council and the freeholder to try and ensure that:
-New bin storage and parking will be added
-there will be no reduction of the communal areas to facilitate the new flats I.E. They're not proposing to carve a chunk out of the second floor hallway to build an access staircase for the new floor(s) above
-the new flats will join and contribute to the existing sinking fund and service charge i.e. their addition will in theory reduce SC for all other units. Make sure owner isn't setting them up separately in their own 'fund' and simply paying the existing kitty an access contribution for use of the lobby etc.
- be clear on what they're adding. If it's just more of the same that's easier to live with than if they're adding luxury units on top of standard units, which would make the old units the aesthetic and financial equivalent of the 'poor door'
-make sure newl units won't have any facilities e.g. Roof garden etc that old units don't have.
Tbh depending on the circumstances I might be able to live with this UNLESS the new flats would mean a lift needed to be installed. Lifts are extortionate to maintain and break down constantly. If a lift is required then scrap what i said above about merging the service charges - you want the development to go ahead on the condition that it's a new and separate development with no shared facilities.