Ceilings and the roof which provide a chance to set traps.
Rats in ceiling cavity.
Set traps inside any drop ceilings and at points like plumbing and heating penetrations where mice can exit the ceiling in the same way as in step 2.
Best part is they leave the nest your house to find water.
Set traps inside a void between ceiling.
There is one very good reason why you do not want rats in the cavity walls and it is the same reason why you should not use rat poison.
Rats can chew their way through.
The rats take them back to their nest have a feast then get seriously ill over the next few days.
The man hole cover has been examined resealed and thames water have also had a look to see if there are any cracks in the pipework they found nothing.
Rats in the ceiling can be difficult to trap and remove especially if your home is one of the few that lacks a proper attic.
Go to bunnings and get some of those blue wax rat bait blocks.
All holes and as much of the cavity walls in the extension have been filled so that they cannot travel up the cavity walls.
You can put them in travel paths toss them in to ceilings or drop them down walls.
Rats can be heard at night in the kitchen and bedroom area.
When a rat dies in your cavity wall it will decompose.
If you have rats in the ceiling walls or attic of your apartment you may find evidence of this behavior.