It could be as small as a log cabin and when entering it would go directly to a special cellar (the crypt) that could be as big as a great hall floor.
Limitations;
To not be used as a super awesome house, crypts should have some limitation like not being able to put most liftable objects inside of it (like containers or crafting tools) - nor build them directly there. It should only allow graves and some decorative stuff like candles or torch posts, stuff on walls, etc...
Use:
People buried in crypts (or around it - it could have a radius, as to form "cemeteries") could get an extra 5% inheritance recovery, going to 65% on normal burial and whooping 95% on state funerals within crypts.
Here are some ideas for crypt entrance: