I have just begun building a garage to house my boat, bikes and tools etc. Well demolishing the old single garage anyway. The dimensions of my boat are 5.8 meters tip to tip inc trailer, 2m wide and 2.2m high (top of A-frame). The building's dimensions are 7m by 4m and 2.8m high (to top of brick work then plus a conventional tile pitched roof). The main door dimensions are, 2.5m high and 3m wide. A roller shutter type door is far superior to any conventional door due to, clearance, security, operation and reliabilty (depending on spec). I have looked at industrial type galvanised roller doors, approx £900 for manual chain, and £1200 for 24vdc electrical control, operted by a push button inside the building. These doors are however suprisingly noisy, and not poular with neighbours. Cardale do rollers also which come out at £700 for and £1100 for a rather primitive remote control type. The one I am probably going to go for is a Hormann or Garador GRP remote type made to measure GRP door. Remote control with rolling code digital technology. Also these doors have a multi pin type locking mechanism which secures the door over it's entire height on both sides, very secure and very quiet. This comes out at around £1200 fitted. Remember! Find out the standard widths of doors before you comence building. The doors can be made any height by adding or removing slats, but the made to measure widths and odd sizes can be very expensive. The whole project should come in at around the £5000 mark with 2 2mx1m UPVC windows and a standard UPVC door also including a big drink for my mate the builder who is doing the technical bit while I labour for him. Much cheaper than the original quote of £10000-£12000!!!
My advice is to use the best materials you can and do the job once. Remember to allow for decent lighting (for late night maintenance) and other elctrical sundries, consumer unit, sockets, armoured cable ,perhaps an alarm? etc (RCD power points are best when working with power tools on wet boats!). Another useful thing in a building like this is water, don't know what I'd do without the mains water and old sink in my current garage, invaluable (saves upsetting the wife by wrecking the kitchen sink aswell!). Try to budget all this in at the start, it is much easier and cheaper to install services during the construction than to try to retro-fit them at a later date.
Sorry about the length, got carried away.
Hope this helps,