For a 20 month old, Fisher Price makes a very nice dollhouse that can be washed easily. Pieces are appropriate for a toddler's hands and mouth (in case of ingestion), are indestructible, and it has lots of nice furniture. They have a "My First Dollhouse" and the "Loving Family House"...both are nice (Loving Family is MUCH bigger and would last your daughter through preschool). Playmobile also makes a nice house that can be taken places...its called, "Take Along Dollhouse" which is nice to have that option with toddlers as they like to bring their houses with them places. Its easy to move around with, with nice classic Playmobile people and furniture.
There are some amazing wooden houses that are beautifully made...they are also much more expensive. If you're looking for one of those, the Melissa and Doug Company sets have lasted at our school for a nice long time and aren't too expensive. Their pieces are classic, the house is very simple and constructed nicely. I do notice that my daughter and the children in my preschool class were more drawn to the Fisher Price set as it has more "realistic" pieces and furniture, so I guess you have to decide what you're looking for. If your daughter is really into dolls, she will move into the Barbie scene or classic Victorian Sets very soon and the first little set you buy will be destined for the basement, sadly, like it did in our house.
Hope that helps!
Jen