I always get the impression she was dreaming about it, rather than having any real intention of doing it. In a way it's like how people now pin all these pictures 'For my future home!' on Pinterest. I'd need to win 10 lotteries to afford all the stuff I have pinned on mine--but it's still fun to dream about it.

The line following her 'plans' for renovating is very telling:

"In the mean time, till all these alterations could be made from the savings of an income of five hundred a-year by a woman who never saved in her life, they were wise enough to be contented with the house as it was;"

Mrs. Dashwood says, "Perhaps in the spring, if I have plenty of money, as I dare say I shall, we may think about building."

and also

"I shall see how much I am before-hand with the world in the spring, and we will plan our improvements accordingly." Even though she and her husband never managed to save anything off a year's income at Norland, she expects to get that far ahead in only 6 months?

Also, when they accepted Sir John's offer in Ch. 5, they took the lease for only a twelvemonth. I'm sure her cousin would be happy to extend the lease, but still those are big plans for a home you might only be in for a few more months after the renovations were done.