From the Monopoly Deal FAQ (scroll down to near the end):
You cannot pay with Houses or Hotels you have on properties because they are not in your bank nor are they Property cards themselves. If you had Banked them, you could use them to pay for things.
answered Feb 13, 2011 at 16:25 LittleBobbyTables - Au Revoir LittleBobbyTables - Au Revoir 12.6k 6 6 gold badges 57 57 silver badges 96 96 bronze badges8,464 2 2 gold badges 47 47 silver badges 75 75 bronze badges answered Oct 3, 2013 at 1:43 51 1 1 silver badge 1 1 bronze badgeHowever, if a player charges you money and you decide to pay with some of the property from your completed set that includes a House and/or Hotel, the House or Hotel must be placed on the table next to your property section until you complete another set and it can be placed on top. You can also pay with your house or hotel card if you choose.
whoa. .we've been moving the houses with the property. Good lord those rules are written poorly and incompletely.
Commented Aug 10, 2017 at 16:51Note that link is a fan compilation of rule Q&As, not an official rules page. So while I do appreciate the answers on that page, I think its entirely reasonable to disagree on corner cases like this as long as your play group establishes the details ahead of time.
Commented Apr 1, 2020 at 20:52The rules (which you can check on this Monopoly wiki page) say that you absolutely cannot play cards from your hand. You can put hotels and houses into your bank though.