linear regression - modelling dependent variables

#1
I'm trying to estimate the value of an apartment, by doing a regression through similar apartments.
The regression model looks now like this
Code:
 lmRob(price ~ ., data = data)
, but this is clearly wrong.

My problem is modelling variables which are depending on each other.
For example floor number and elevator (is a dummy variable 0 or 1)

If the apartment is at ground floor, the existence of a elevator is irrelevant. But if the floor number is higher, the importance of the existence of the elevator is also higher.

How could I model this in my regression model? Any suggestions?

Thank you!