I need to run a cable from the house to the grannyflat out back to use ADSL2+ but looking on the net people are advising that the extension telephone cable running to the modem should be under 3 meters. Why does it have to under three meters? What's under the floor within about 3 meters that alters the net with a longer cable? The extension phone lead I need to use is 15 meters long. More importantly, I need my router in the granny flat where I'll live, so what can I do about it?

BAD idea.Phone extensions are NOT advised for any broadband installation.
Better would be to run Cat5 from the router AFTER the ADSL connection.

Isn't a cat5 a cable that runs to the laptop? I have a wireless router