First, determine the required fire flow for the house(s). Check the local fire code. For example, the Uniform Fire Code requires 1,000 gpm for houses less than 3,600 sqft. A reduction is allowed (subject to the AHJ) for a fire sprinkler system.
Second, AWWA Manual M-31 (Page 21) indicates the total demand should be the required fire flow plus (either max day or max hour demand - whichever is greater).
Consider time of day and time of year fluctuations in the water supply system flows and pressures. These are not necessarily trivial.
Jim Feld, P.E.
Fire Protection Engineer
Fairfield, CA