There's a thing called leasehold whereby you own the building and lease the land usually for 99 years after which it returns to the freeholder. It's one of the reasons that the US embassy in London moved from Mayfair to Nine Elms. It was the only US embassy in the world that the US government didn't own, the freehold belongs to the Grosvenor family (i.e. Lord Grosvenor). When the US tried to buy the freehold the Grosvenor family refused but agreed to a 999 year lease in exchange for the return of 12000 acres of Florida that was confiscated from them after the Revolutionary War - yes, they've been landowners for a very long time! I think the US made sure to buy the freehold of the new site at Nine Elms (they sold the remainder of the 999 year lease in Mayfair for an undisclosed sum) 😀