Nobody thinks about the water shutoff until a pipe lets go on a Sunday night, and then it's the only thing anybody can think about. You're the one with two houses, so the question isn't just whether you can find it. It's whether the kids' house and Mom's house each have one that you, or whoever is standing in the kitchen with you, can actually turn.
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Find the valve first
Start where the water comes into the house. In many homes that's the basement, a utility room, a crawlspace, or the wall where the pipe enters near the water meter. Some homes have a second valve outside, near a hose spigot. Walk through each house and write down every valve you find, including the ones that look like they belong to a sink. Some of them will.
Turn it once, gently
Older gate valves, the round handles that take several turns, can seize after years of sitting still. A quarter-turn lever, the kind whose handle lines up with the pipe when it's open, is usually easier to trust. Test on a dry day, not in the middle of a leak. Close it slowly, watch the faucet nearest the valve for a few seconds, then open it all the way back up and check the valve for drips at its base. If the handle is stuck, don't force it with a wrench. A broken valve can turn a small leak into a big one, and that's a call for a licensed plumber.
Check the street side too
Some water service has a second shutoff near the street, sometimes under a lid in the yard, that needs a special key. That one usually belongs to the water utility, and you're generally not supposed to operate it yourself. Ask your water provider where their responsibility ends and yours begins, and whether they have a process for a valve that won't close. Put their phone number on the same card as the valve location.
Make both houses match
The point of a split week is that whoever is on duty can handle a problem without a phone call across town. Compare the two houses side by side. Is the valve in an easy spot, or behind stored boxes? Does the handle move freely? Is there light in that space? Is the wrench or key kept with the valve, or does someone have to go hunting for it?
Then decide who checks it. Name that person in writing, and keep the same card at each house. Keep the card short: the valve location, how to close it, the utility's phone number, and who to call first.
Questions to ask before the leak
Ask a plumber or your water provider these before there's a problem. Which valve shuts off the whole house, and is it working? Is there a separate shutoff for the outside faucets, and does it drain them? What does the valve at the street need, and who is allowed to use it? If a handle won't turn at all, what is the repair, and who schedules it? Write the answers on the card, not in a text thread you'll have to scroll for later.
Once a year, on a dry day
Put it on the calendar you already keep. Close the main, check the nearest faucet for drips, reopen slowly, and check the valve again. If anything changed, such as a stiffer handle or a new drip, update the card and make the call while it's still a small problem. Wiping the handle and clearing the path takes about five minutes, and it keeps the valve from becoming the surprise nobody planned for.