Many of these countries have Euro. Eurostat does not disclose detailed methodology, this is what I have found:

The House Price Index is part of the so-called Euro-indicators that are designed to give a general overview of the economic situation of the euro area, the European Union and Member States.Weights are usually compiled using data from National Accounts, Household Budget Surveys, Construction Statistics, etc. Weights for HPI and OOHPI categories are revised yearly and released with the data for the first quarter of the current year.Data are not seasonally adjusted.