Hi I will try and help.
when there is the build up of water does this run up to and against your neighbours garage?
also, has this always happened, or has it been happening periodically during the approx 20 years period
sorry, on the last point I meant to say has it always happened, or just recently started happening?
does your land sit higher than your neighbours? and do you know if your deeds contain any rights of drainage over your neighbours land?
also do you know when the garage was built. was it within the last 20 years of you living there?
also, am i right in presuming that you use the land for its natural purpose. It is presumably residential, so you don't conduct commercial activity at the property?
and was the drain on their side near to the flooded area on your side of the property?
Ok thanks. Well if we start from first principles then a person is entitled to be protected from anything escaping from a neighbouring land onto their and causing damage (i.e. chemicals, animals, excessive water etc). If this happens they can claim under the law of nuisance (which has both common law and statutory legal "arms") and claim for loss and damage. However I think your neighbour would have a long way to go in both proving a case against you as the landowners. Firstly a Court would look at the cause of the flooding which in this case appears to be a natural phenomenon and not something you are doing which is not the intend use of the land (i.e. if you kept cattle on the land for example). There would have to be an expert produce a report to establish exactly what the cause of the flooding, but i suspect it it more likely to come from your land than up through the floor of the garage. Secondly if would look at what is reasonable in terms of what you could do, ad what they could do to prevent this flooding from happening.Again if it is significantly costly or problematic then a Court would not expect you so take excessive measures.
There is a forum that you could look to to assist in helping resolve the situation, other than a County Court, and that is the agricultural lands tribunal (despite its name it does deal in all types of land)
If course the first point of call is to try and mediate with your neighbor to try and resolve matters amicably
I trust this helps. Do ask any follow up questions.
Well, that is the difficult one I am afraid. Each case would be decided on its facts, the type of repair, disruption to property and life, the percentage chance of the fix actually working, etc. Really very difficult. I would say that a Couple of hundred or a thousand may be seen as probably reasonable (particularly given that to fight it would cost more in legal fees) but it is difficult to give an accurate answer.
I hope I have helped you. If so please leave me positive feedback so I can be paid for my time. The question wont close and if you wish to ask any further follow up questions you can come back to it any time