The rest of the house has hot water. Only lukewarm water in bathroom

Are you sure the shutoff valve in that bathroom hasn't been partially closed? Possibly by a child. They tend to be low on the wall and acessible

Hi,
Adding a whole new complete bathroom to an existing water service(main) without calculating fixture units would mean an upgrade on water heater. Or there could be water lines that is crossed between hot and cold near the new bathroom.

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Adding a whole new complete bathroom to an existing water service(main) without calculating fixture units would mean an upgrade on water heater.

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This doesn't make much sense at all.
The OP never said anything about adding a bathroom so I assume he's talking about a existing one.

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Or there could be water lines that is crossed between hot and cold near the new bathroom.

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where did you get that there was a new bathroom?
A better explanation would be that one of your single handled faucets has developed a crossconnection in the cartridge allowing cold water to mix with the hot. To check shut off the hot and cold supplies to the lavatory and run the hot water from the tub valve. If you get hot water after a minute or two the lavatory faucet's at fault. If it still runs cool out of the tub I'd be giving a hard look at the tub valve. Let me know what you find. Good luck, Tom