Is this legal?

[gasert]

Hi, my next door has been turned into a student house
It's not registered as an hmo, I checked.

There are 5 single students in separate rooms, and only one bathroom with a toilet in. No additional toilet (and no space to create one). No living room, just a tiny kitchen.

Is this legal?

No but good luck getting council to act.

It might be, local councils have differing criteria. You should be able to check online or by a quick phone call. Regulations tend to be more focussed on safety(smoke alarms and fire exits) than comfort though.

Is the owner Rachel Reeves?