A home with an accessory apartment, sometimes called a basement unit, a garage suite, or an in-law unit, often has one water supply serving two households. Sizing a treatment system for that home means counting both households, not only the owners. Use this guide to decide what to tell the installer and what to ask them to write down.
Confirm how the water is plumbed
Find out whether the apartment has its own meter or shares the main line with the house. Trace the cold supply to see whether the treatment unit sits before the split, which means it serves both units, or after the split, which means it serves only one. Put this in writing for the installer. The location of the equipment decides which household receives treated water and which does not.
Count the water each household uses
Ask the installer what daily water use they assumed for each person and for each household. Many sizing methods start from the number of people and the hardness or contaminant level of the water, then add a margin. Ask for the figures they used and where they came from. Then compare those figures with what you know about the occupants. A household that works from home, runs a laundry most days, or hosts overnight guests uses water differently from one that is out most of the day. Give the installer your best estimate of how each unit is actually used, including any tenants whose schedules differ from yours.
Test the water that feeds both units
Ask for a test of the water at the point where the treatment unit will connect. The test should reflect the supply that both households receive. If the apartment has a separate well or a separate meter, test that supply on its own. Do not assume one test covers both if the plumbing is different. Keep the report with the sizing paperwork so the design can be checked later.
Decide who uses what and who services it
If one treatment unit serves both households, decide in advance who arranges service, who pays for it, and how each household reports a problem. Check whether the lease or the property manager already covers equipment, and put the answer in writing rather than relying on a verbal understanding. This is a practical arrangement question, and a written note prevents most disputes before they start.
What to write on the quote
Ask the installer to note the sizing assumption for each household, the test used, the point of connection, and the service responsibility. A sizing plan that names both households and their assumptions lets you check the design when occupancy changes. If the apartment is later rented to a household of a different size, the same note tells you when the system needs a second look.