The novel starts out being set in Sussex County in England but then the Dashwood women must relocate. We learn of the initial setting of the novel in the very first couple of sentences in the book:
The family of Dashwood had long been settled in Sussex. Their estate was large, and their residence was at Norland Park. (Ch. 1)
Norland Park would be the name of the grounds on which the Dashwood estate is situated on, and their actual manner house is called Norland. The county of Sussex is located just south of London.
When Mrs. Dashwood's husband dies, her step son John Dashwood inherits Norland and, thus, the four Dashwood women must find a new and inexpensive place to live. One of Mrs. Dashwood's other relations offers them Barton Cottage to rent, which is situated on Sir John Middleton's Barton Park estate in Devonshire county, otherwise called Devon county. Devonshire is located about three counties west of Sussex and south-west of London. Barton Park and Barton Cottage become the setting for the rest of the novel, except for a moment when the two elder sisters travel to London. We learn of their new prospective location in the fourth chapter in which we learn of the letter that contained "the offer of a small house, on very easy terms, belonging to a relation of [Mrs. Dashwood's], a gentleman of consequence and property in Devonshire" (Ch. 4).
We can also deduce the time period the novel is set in based on certain allusions in the text. An allusion is when one author refers to another author's work. While still at Norland, when it starts becoming clear that Edward and Elinor are fond of each other, Marianne makes a comment to question her sister's choice in Edward by criticizing his reading skills. Mrs. Dashwood retorts that he would have read much more smoothly had Marianne given him something simpler to read than the poet Cowper, as we see in her lines:
He would certainly have done more justice to simple and elegant prose. I thought so at the time; but you WOULD give him Cowper. (Ch. 3)
The poet William Cowper lived from 1731 to 1800 and was therefore a contemporary of Austen's. In addition, Cowper was one of the Romantic poets and one of Austen's central points in the novel is to show just how dangerous it is to act upon the sensibility that the romantics taught rather than using one's sense. Hence, based on this allusion to Cowper, we know that just like her other novels Sense and Sensibility was set in Austen's own time period of the late 18th and early 19th centuries. We also know that the book was published in 1811.