for a large rural house... yes, it would be an average size.
consider it may be used for fuel storage, general storage (bikes etc), workshops etc...... and god forbid, maybe even a car........... and with those dimensions there may be a usable loft as well.......
but remember, you dont have to build to those exact measurements.....
the cert of compliance you will receive will state 'opinion on substantial compliance'... and also 'in so far as site works are complete'... this gives you almost carte blanche on the garage...