How can I market size the number of empty houses in a country? As in how many houses/appartments are currently empty.

I believe what you are asking is the % of homes in a country that are vacant/not occupied

One way to approach this question -

Estimate number of houses

1. Look at overall population
2. Divide population into number of households (e.g., 3 or 4 people per household on average)
3. In a perfect world 100% of households have houses
4. Now you can apply discounts as needed - for example, if you are only looking at single family homes then there will be a discount as a lot of people living in cities live in condominiums
5. If you are taking all types of accommodation then you will apply a positive multiplication factor as condos tend to have fewer than 3 or 4 people in them

Now you want to look at vacancy rate

1. Factors that affect vacancy
   1. Oversupply of homes
   2. Declining population
      1. Aging population
      2. Net migration
   3. Urbanization

Look at factors that impact the country in question

1. E.g., if it is oversupply you can look at why (low interest rates leading to building boom and ghost towns, or people owning multiple homes, or new towns being built etc)
2. Either apply a relevant discount factor to the total homes (e.g., 5% homes are vacant because of multiple home ownership - you will have to have some rationale as to why)
3. Or try and calculate what these might be if you have relevant information - (e.g. number of new homes built vs population growth rate)

Hope this helps,

Udayan

(editiert)