The novel begins at Norland Park, a fictional estate in Sussex. Austen doesn't describe Norland in great detail, only that it was "fine old house in the county of Sussex". Having spent most of her life neighbouring Hampshire, Austen would have been fairly familiar with Sussex and we know that she spent some time in Brighton and Worthing.
When the family home is entailed away after the death of Mr Dashwood, Mrs Dashwood and her daughters move to Devonshire and take up residence in Barton Cottage "within four miles northward of Exeter". The cottage belongs to Sir John and Lady Middleton of Barton Park, which is believed to be a fictional version of Pynes House.
A large portion of the novel takes place in London, where Marianne and Elinor stay at the home of Mrs Jennings on Berkeley Street, off Portman Square.
Emma's world is a small one and most of the novel takes place in the fictional village of Highbury in Surrey. Austen describes Highbury as a "large and populous village, almost amounting to a town" and tells us that the village is 16 miles from London, nine miles from Richmond and seven miles from Box Hill. Highbury is most likely an amalgamation of several places, but Austen may have used Letterhead as the template.