I would think about just pouring a slab adjacent to the existing one, but out of the way of the driller's access for your booster pump / filter setup.
FWIW: Our main pump house has a lift off roof, walls that lift off, and a power panel that is attached to the slab and not the walls. All are corrugated iron for fire reasons. Our sediment filter is in with the main pump house prior to the water heading up to our main tanks that gravity feed the ranch. It all fits, just. The piping is so convoluted that if anything ever broke, we would probably have to rip it out and start over. The pipes are insulated, which seems to be enough in this climate, though our other well has foam insulation in the roof. I am reluctant to put insulation in the walls due to rodents.
The lift off roof has come off only once, in sixty or so years, in a recent 60+mph windstorm that toppled more than a few trees locally. The tractor had the roof back on in about three minutes with a pair of forks. I figure having the roof come off beats having the structure pull on water pipes.
All the best,
Peter