I have a fairly generous goat shed that PH built , it is concrete foundations/base, and 10ft by 18ft. Same size hayshed, all because of the lengths of wood available. The goat shed is divided into 4 pens of 5ft by 6ft, with a central area in which there is the entrance, milking area, feed bucket, water tap etc. WE have put electricity in, and I think that is essential for winter (milking kidding etc). My tw GG nannies share, as do the two BT kids(now goatlings). On epen will be for kidding, once my wether kid/buckling is in the freezer.
Lots of ventilation above the had height of the goats, and large mesh doors for when they are inside.
However people use B&Q sheds successfully, you can get cow mats of thick rubber I think, and you could lay an extra layer of flooring to extend the lifetime. I have the equivalent of kickboards (as for horses) in the pens, and I also deep-bed them. In the summer cleaning out every months or so, with daily topping up any wet areas and taking out any droppings (usually from where they sleep at night). In the winter no mucking out between Nov and a week or so before kidding. But this year we had to take some of the muck out from one of the pens, as my BT type nanny was poo-ing into her waterbucket every day, the muck layer had grown too much in her pen. I normally would have had a last big muck-out end of November, but as winter came early that never happened... but they are warm, no extra heating required and they are not rugged at all.
If your OH is good with wood, look at the Fiasco farm website for construction of a milking stand!