In Monopoly, suppose you own the brown color group ("Old Kent Road" and "Whitechapel Road"). Now further suppose you have built a single house on either one of these two properties (eg Old Kent Road).
Can you now take out a mortgage on the other (Whitechapel Road) one or do you have to sell the house (on Old Kent Road) first?
You must sell the house(s) before you can mortgage the property. From the rules, page 4 (emphasis mine):
MORTGAGES… Unimproved properties can be mortgaged through the Bank at any time. Before an improved property can be mortgaged, all the buildings on all the properties of its color-group must be sold back to the Bank at half price.
