I would not worry too much about your estimation error, a 50% estimation error would not be a big deal, you just need to get magnitude of a number.
Interviewer don’t want you to get the exact number, they are more worried about your reasoning.
Looking at your estimation process I think you should pay attention to the question: you are asked to estimate the number of houses (including single family houses and condos).
From your calculation seems you estimating the number of homes, (same as estimating the number of households), rather than the number of houses.
A different approach to this question could be the following:
- Avg. household size: 3 people
- People leaving in single-family house: 70%
- People leaving in condos: 30%
- Avg. number of apartments per condo: Avg. number of floors * Avg. number of apartments per floor
- Avg. number of floors = 5
- Avg. number of apartments per floor = 5
- Number of single-family house = Total population * people living in single-family house / household size
- Number of condos: Total population * people living in condos / households / Avg. number of apartments per condo
Please reach me out if you want to discuss it further,