How to deal with data that is only available for a certain category

I am working on a house pricer model, and I have a feature with values 0 or 1 to indicate if the rent price is capped by the government or not (houses with capped rents sell for much lower on average).

when the rent is indeed capped, there is a second feature with the cap value.

How do I deal with this second feature ? knowing that it's missing for more than 80% of the data ?

Thanks in advance.