damp extension

[Letdown]

The letting agents who were looking after my property have gone bankrupt - i have found the property to be in a terrible state.
There are tenants still there and i need to get things sorted.
The extension consisting of a bathroom and kitchen are completely damp - it is an old house and the builder says no cavity wall - the gardens of neighbours that back onto the walls are probably 4/5/6 foot up the wall - builder says this would not be allowed in this day and age - he thinks this is adding to the problems. What i want to know is:
Why would the walls be so damp
What can I do
How long will it take
Will the tenants have to move out
How much will it cost
Basically any help would be very much appreciated.