In Chapter 14, when Willoughby is waxing poetical on the perfection of Barton Cottage, we read:

"How often did I wish," added he, "when I was at Allenham this time twelvemonth, that Barton Cottage were inhabited! I never passed within view of it without admiring its situation, and grieving that no one should live in it. ... "

This is late October/early November (by my reckoning), so Barton Cottage was empty almost a year (at least) before the Dashwood ladies moved in. I had been thinking that Sir John had lately lost a tenant, and coincidentally had a house for Mrs Dashwood. But if the house were empty for several months, I begin to wonder if Sir John was just waiting for a tenant w/ children old enough for dancing and other parties. :-)