What would happen to the palaces and castles if ....

[HouseofWindsor]

Monarchy was abolished?

Windsor castle is looking magnificent today along with the Great Park.

Sheltered housing ? President residences ?

I don't think any of the royal residences would become offices or fall into disrepair. Versailles and the chateaux of the Loire haven't, and not have any former royal residences in Germany. Even the palaces of Tsarskoe Selo are pretty well preserved.

Buckingham Palace needs money spent on it and nobody really wants to spend the money.
Chunks of it is office space anyway so you could probably let people see the nice parts of the offices will stay as they are.

True. And come to think of it, I'm not sure Buckingham Palace is quite up there with Versailles or Neuchwanstein (sp?) in terms of spectacle!