Hi Guys
First thing , try not to disturb them, 2nd thing proof any holes and where poss find the way they are getting access to the house and proof it all off and remove any obvious food source and water as much as possible. they could be climbing a tree and running across the guttering or roof getting in under a tile or have come up the sewer pipe into the void between the inner and outer walls or through a hole in the wall check all areas for signs and smear marks.
Rats carry loads of infectious rubbish ensure you wear gloves etc when undertaking the work and if there is evidence of activity in the kitchen or living areas sanitise work tops etc and block off access holes etc
Assuming the activity is throughout parts of the house, the roof void or loft is the prime area for the nest etc so inspect to locate best point of activity if we say it is the loft area which is secure and free from disturbance from non target species of animals.
Rats have to have a drink, cover open water tanks, reduce any poss available food source and first point of call use mechanical traps, always fix into position with a piece of wire if they are skitzo, they do not like change , leave things as normal as possible pre bait the traps on non catch mode with corn and put a drink station in an area they use and let them settle down, leave the water in situ and set the traps to catch, make sure the mk4 fenns (these are brilliant) are installed in the runs, if needs be put a smear of peanut butter on the trap and sprinkle wheat on this and just leave it to adhere to the plate, it is a game of patience, always watch to see any new runs coming along and adjust traps accordingly YOU MUST CHECK THE TRAPS AT LEAST TWICE A DAY. trapping takes longer and is time consuming, but you see the result and there is no risk of a dead one popping off in a wall cavity and stinking the place out.
All mechanical traps must be covered by a cage to stop non target species getting caught, this even applies to the loft area as well as all other places, this would be the first point of action to remove the problem, when you have removed the rodents any obvious runs place some fine sand or flour on the runs and inspect regularly to ensure you haven't missed any.
if you keep getting a new flurry, you need to find the entry point and seal it off - very important as they will keep coming back, could be just running up the sewer into a broken pipe, unless you find the access point it won't stop.
If you resort to rodenticide, make sure it is a safe area to use (preferably in a tamper resistant monitor) and no non target species can get to it and importantly make sure it is fixed in position as they will remove bait and stash somewhere and you think they have eaten it all, the amatuer rodenticide such as bromodiolone, Difenacoum are two feed applications, you need to inspect and refill on a regular basis and if the activity has not greatly reduced or gone after 28 days , call in a professional. you may need different bait formulation to complete eradication and further proofing investigation
Hope this of some help
Phil