Symbolism, Imagery, Allegory
Think back—way back—to the beginning of Clarissa. The whole kerfuffle started because her rich granddaddy gave her an estate—and a dairy-house. In one of the first letters, we find out that Clarissa is often away because she's "absent at my Dairy-house, as it is called, busied in the accounts relating to the estate which my grandfather had the goodness to devise to me" (2.6).
Oh boy. Few problems here: (1) she's got something that the other kids don't have, and (2) she has her own house and her own money. In fact, she repeatedly says that all she wants to do is to hang out on her own estate and be left alone. To her, the dairy-house and estate represent self-determination and freedom: the ability to make her own choices and, crucially, have her own money.
Obvs, her family won't stand for that. So, in the end, she settles for another kind of house: a coffin.
Remember all that weird business about Clarissa ordering her own coffin? She repeatedly calls it a house, and when she dies Belford says that she was "removed […] in to that last house which she had displayed so much fortitude in providing." In the end, Clarissa does get to make her final choice and provide her own house. Too bad that there's no exit door.