Tanis8472
Guru
Not sure what would be best. I have a wooden shed, but none of the bikes in it are worth very much and I have some security measures. You need to check the situation with planning though. I think 8' x 6' is the biggest you are allowed without permission. The situation may vary depending on location.
Dont need permission unless it takes up more than 50% of the original house build land.
Height is more of an issue normally.
Couple of links
http://www.planningportal.gov.uk/permission/commonprojects/outbuildings/
http://www.planningportal.gov.uk/permission/commonprojects/outbuildings/miniguide