Capping of shared water supply pipe if the tee is under an extension

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My property is the last of 3 houses on a shared supply. We are considering trying to get our own supply fitted for various reasons however I have a query about capping off the old supply. I understand it should be capped off at the tee to avoid a dead leg however the previous owners of the house attached to us, 2nd on the supply, had built an extension which may have been built over the supply pipes (at the time the builder definately built over the gas mains which also run behind the houses as we had to have our garden dug up for them to be rerouted when discovered and their meter moved outside so I would be surprised if they bothered to move the water). If this is the case is there anyway of capping these pipes safely or is this just not feasible? I will have a chat with new neighbours to see if they know but just wanted to check on worst case scenario or if it might just be a no go. Thanks.