 <p>My understanding is that the <strong>user</strong> can extend localstorage but the website can't (by design). You simply need to catch the error in Javascript and show the user a dialog requesting they increase their storage limit - preferably providing some instructions for major browsers.</p> <p>EDIT: Perhaps not so simple. It seems some browsers don't allow the user to increase the storage size. <a href="http://code.google.com/p/chromium/issues/detail?id=58985" rel="nofollow">Google seems convinced the localStorage API doesn't scale well to large files and developers should consider IndexedDB instead</a>.</p>