Luckily, the ever-excellent but, alas, all-too-infrequently posting Nosemonkey has looked into the matter with his customary attention to detail—including debunking the 84% figure.
No one agrees on how much legislation and regulation stems from the EU. The 9.1% figure stated by the House of Commons Library is too low, as it only covers Statutory Instruments, not ALL laws; the higher figures of 84%, 75% and even 50% claimed by the likes of Hannan, Farrage and Cameron are based on miscalculations, misunderstandings, or sources unknown, and often derive from parts of the EU other than just the UK - and so with no hard evidence to support them must be dismissed as either too high or inapplicable to the British situation.
What is the true figure? No one knows. So any claims that state hard and fast percentages should - if we’re being intellectually honest - be treated with equal suspicion.
If no one knows—including the Commission that proposes the laws* and the EU Parliament that passes them—then how can they do a cost-benefit analysis on the laws that they foist onto us?
As I pointed out over there, Commissioner Gunter Verhoegen, in an interview with the FT, estimated the benefits of the free trade area to EU businesses, to be €200 billion a year—but the costs of EU regulation, to those same businesses, to be €600 billion per annum. Another commenter, however, maintained that even Verhoegen's word could not be taken at face value (no surprises—he is a politician after all—but one would have thought that Verhoegen, at least, might have a good idea).
In fact, this conclusion is extremely worrying for anyone that cares about good law. After all, many of the EU laws passed—especially in the area of Environment—are aimed at getting people to change their behaviour through economic means. If no one has done a cost-benefit analysis on these laws, how can they know if they will, indeed, change said behaviour?
Come to think of it, this fucking insane lack of cost/benefit analysis would explain an awful lot about some EU laws...
UPDATE: thanks to commenter Jonathan Miller for a clarification.
DK, you seem to have confused the origin of the various legal instruments.
Statutes (primary legislation) and statutory instruments (secondary legislation) are UK 'laws'.
Regulations and Directives are 'laws' enacted by the EU.
The UK enacts EU Directives by way of statutes and statutory instruments.
Regulations are enforceable in all EU states, overriding national law. They don't need to be transposed into statutes/statutory instruments.
...so the calculation should include Regulations, Statutes and Statutory instruments, but not Directives.
Yep, mea culpa: it's been a while since I refreshed my memory about this shit.
* Yes, I know that, technically, the EU does not make "laws"—only Statutory Instruments, Directives and Regulations that need to be enacted into the statutes of member states. However I am calling them "laws" merely for the sake of convenience.