1. Consider water. Don’t add bathrooms without looking carefully at your water supply lines and the capacity of your water heater. A smaller tank or one with lengthy recovery times may not meet your needs. This is especially true if you select a new 80-gallon soaking tub and you have a 50-gallon tank. Consider a separate tankless or tank heater for your second story, or a larger tank with a recirculation pump that keeps hot water right at your taps. Also consider how large your street-side water supply lines are. If you add enough fixtures, the plumbing code may require you to upgrade to a larger supply line. And if you still have galvanized piping, this is probably the time to take it all out.