I have the same. My neighbour and me have a garage each joined together and the guttering runs along the front and back, and down the downpipe on his side. I want to put a water butt in my back garden effectively cutting in a running outlet (or downpipe section) in my side of the garden.
The way I see it, and the way I am going to do it is;
Install the running outlet and downpipe, and go directly into the lid, or the side of the butt, however you want to do it. Then when it rains and your butt is full up, the water will then collect up the downpipe, only when the water level gets up to the guttering it will drain away along the rest of the guttering back down the original/official downpipe into sewer/soakaway.
Its only a section about 18 inches or 2 foot max from the top of the butt to the guttering level, so not much water sitting in there.
I see no other option, except for it to overflow straight onto the lawn!
It would only work if the lid and the pipe going into it were totally sealed otherwise it would just pour out. And you might want some form of strainer in the gutter to stop leaves, grit, etc. going down there.
Okay - so how to do this - well one way.
The key item you will need is a side inlet float controlled switch for a toilet cistern - this will shut the water off when the tank is full - all you then need is to connect this to the gutter or your neighbours down pipe depends on how keen they are to having some extra piping run off.
If you are freinds with them and it is not so far use a standard but connector - the actual location does not have to be at the water but height as the control is via the float.
If you cannot use the exisitng downpipe and some tubing - make your own down pipe and terminate it to allow you to connect your now float controlled inlet to (remember if you have reducers which may get blocked - best to have the reducing end near to the water but height so you can clean it out easy. Solvent weld pipework and access plugs can be great for blocking up a standard downpipe water butt connector.
Hope this helps
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local