I've done the same but mine is 7.5m by 8m and it pissed the neighbours off too

When I spoke to the council around my way they said roughly the rules are -

if its less than 50% of your total land.
doesn't exit on to/across a public highway.
Roof height under 3m if flat or 4m if pitched.
7m away from the house.

if it contravenes any of these it needs planning but if not you only need to apply for building regs.

HTH