Finnish City Inaugurates 1 MW/100 MWh Sand Battery

It seems to depend on having a network to distribute heat, which is something that you have to take into account... basically when you create your town ?

Or is there a way to "retrofit" district heating into houses with their own gas boiler or heat pump ?

They already have district heating.

Over half of Finland’s population lives in buildings connected to district heating.

Finland traditionally mostly used oil. If you have traditional water circulating radiators or underfloor water circuits it is pretty easy retrofit, just need to swap boiler with heat exchanger. Still, in many cases cost of infra is pretty high so heat pumps are often better options.