I would have thought that falls under permitted development. On looking at it the only thing of concern would be the 50% of the garden rule.

My wife didn't complain when I built my two bay garage with mezzanine, she suggested I put a bed up on the mezzanine and to build a pulley system so my meals could be pulled across.

Sounds to me like you have a great lady thereMy wife didn't complain when I built my two bay garage with mezzanine, she suggested I put a bed up on the mezzanine and to build a pulley system so my meals could be pulled across.
A very nice workspace but be careful when welding in there as that white reflective surface will play havoc with reflected arc. Worst case of arc-eye I ever had was welding in someones garage made of white lightweight blocks. Caught me out and have always been very wary since.
. Re hatched the dimensions to 16 x 20 foot and it's just under the 30 sq mtrs mentioned .. Will spend some time tomorrow checking local planning regs to see if this is Ok here in south wales or if I have to have the smaller cheaper 16 x 20 jobby