[Closed] Building garden gym / office - ideas?

I'm planning to build a building in the garden where I can have a rowing machine, a few free weight and maybe a desk for mini home office. I'd like something better than a summerhouse shed but don't want to spend a fortune. I've seen the pre-fab ones which look a bit pricey. Also interested in something alternative like timber frame and straw bales but I imagine it will be expensive. Also seen a couple that look like shed with insulated render and basic fibre glass roof which might be good?

All help is appreciated