I had a small pump house that looked like a wishing well when I moved in. Very tacky and falling apart. I checked with the local building official, and found I could build anything under 100 sq ft without a permit, so that set the basic design. I went with an 8x12 shed on a 4" slab, with a hip roof, shingles to match the house, a couple of windows and a 4' door. Cost me just under $1000, I guess. Heating is not a problem here . There's enough room for the water treatment system, lawn equipment, space for a workbench, etc.