FOR THE UK ONLY: Rainwater drainage from neighbouring property ?

First go and look at the plans for the garage at your local Council Planning Department, establish where the drainage should be. This will have had to have been cleared by the Environment Agency as being suitable. The disposal of rainweater is dealt with by your local water company, whoever you pay your water rates to. While you are at the Council ask for a photocopy of the plans, they will charge you a small fee, but it could save you having to return at a later date. It is quite legal to have rainwater go in to a soakaway. I actually have the same set up. If you think there is any damage being caused to your property contact your insurers, they will arrange for a surveyor to check it out. Hope this is some help.