The modern water softener is designed to fit into an under counter
cupboard. Typically sited in the kitchen, utility room, garage, outside and a
few other places downstairs.
The usual installation involves softening all the water in the house from
the rising main stopcock with the exception of a new hard water at the
kitchen sink if possible and sometimes the garden tap as well.
In general terms, put the machine where it is easy to get to for filling with salt and run all the pipes back to
the rising main stopcock.
That said, putting it in the kitchen sink cupboard regardless is just lazy
and usually inappropriate because the sink cupboard is typically the busiest cupboard in the whole property and another location is usually better,
even under the drainer would be better.
A slightly harder installation is better than a hard to access water softener
for you the owner.