I heard that an extension/conservatory has default planning permission
> after a certain period of time, even though originally constructed
> without planning permission and outside permitted development. Is this > true?
No. It's just that breaches are not pursued after four years. Not quite the same thing. Unless it's a listed building - in which case breaches are pursued for ever.
If so, how long is this period?
>
> What impact does such entensions/conservatories have when the house is > up for sale?
Oh a fair bit. If it doesn't have pp, this may still make it more difficult to sell and expect a careful eye to be passsed over it with respect to building regs, too.
How do you prove that they have default planning
> permissions?
You don't and with difficulty.
Will it be any different after the proposed seller's pack
> come into action?