How much property is optimal?

Inheriting real estate brings more than just a land registry entry. It presents heirs with a far-reaching decision: Should the property remain in the family assets, perhaps as a source of ongoing rental income, be used personally, or is a sale advisable to free up capital and restructure the asset allocation? 

In principle, real estate ownership fulfills two functions with regard to wealth accumulation. On the one hand, alongside traditional financial products such as stocks, bonds, and ETFs, it serves as a diversification component and, as a tangible asset, offers security. On the other hand, it provides an additional source of returns, for example, through ongoing rental income and capital appreciation. Including it in the asset allocation is therefore certainly sensible - but how much is "optimal"?