another vote for slug pellets (the small blue ones).
I would start by having a look round the outside for the point of entry. Slugs can get under doors. Broken brickwork or airbricks will let them in. Have a look under the floor as well. If it is damp there could be hundreds. Lean-to shed or outside WC will encourage them. Flint, drystone or gappy, unpointed brick walls will provide homes for them.
Scatter slug pellets where they are out of reach of pets, and, over time, they will take the population down. You might not think they are working at first , because other gastropods walk in from nearby. Replace after rain, or scatter weekly. They need to be scattered so that wherever a mollusc happens to wander, it will bump into one and have a nibble.
If you have ivy or other climbers on the house walls they will be living there. If you have a source of damp, including drips and leaks, this will give a comfortable environment.