Scriptpunk asked this on the Bang2writers page on Facebook:
A Q for You! Do you have any idea when book copyright goes into the public domain? ie. Jane Austin, Charles Dickens, Bronte Novels - would you need to get permission from the estate to adapt or are they in the public domain therefore open to reintepretation al a Autin's Emma as 1990s Classic Clueless?
UK copyright lasts the author's lifetime plus 70 years. So, the works of Shakespeare, Chaucer, The Bronte Sisters, Jane Austen, Charles Dickens and all their mates are up for grabs. You don't need to ask anyone to adapt their stuff; you can just do it. It's pretty easy to see why Victorian Literature (and older) is so popular then when it comes to adaptation.
Dead authors don't necessarily mean you're home-free either then - for example, Roald Dahl may have been dead since 1990, but his work is still in copyright. Make sure you check how long your chosen author has been dead for!
By contrast, you MUST buy the film/TV rights of any living/recently deceased author before you can adapt their work. It is generally unwise to adapt something without the author's permission and try and seek the rights later - for all you know, the rights may already have been bought, then in which case you've just totally wasted your time if you actually want to take that script to the marketplace.
In addition, it's worth remembering film and TV rights are sometimes sold even BEFORE the book has published! Other times, the novelist wants to adapt their OWN work. That's not to say spec adaptations are blown out the water - sometimes novelists will enter into collaborations with screenwriters in a bid to take their work to film and TV, but you really need to do your research and like everything in screenwriting, create those relationships to get projects like that off the ground.
Another thing worth remembering - all this copyright stuff can depend not only WHEN it was written, but where you live, too! Here is a list of countries and their copyright laws that's worth looking at.