You will be better off with a seprate stand alone system.

thanks!

this is a "granny flat"--we would like separate controls for the lower level.

How large?

If small, then a mini split HP might be better for both comfort and control.

IMO

that looks like a great idea! are they quiet? do they also dehumidify? do we close off the ceiling vents? is any of the installation a DIY project? the apartment is about 400 sq.ft., with a separate bath and bedroom. thanks

"...finding it very humid and cold during the summer..."

You can get a dehumidifier to resolve the humidity problem. It will also help warm up the basement. Do you think the low temperature in the basement is being caused by the house AC? The duct work may be leaking cold air. Inspect and seal all joints with mastic.

A mini split is not a DIY project. It is a good solution but it may be overkill for a 400 sq. foot basement. It sounds like you don't need any cooling. It does not take many BTUs to heat a basement. You may be able to add a zone to your existing furnace. See if that is a viable option before spending money on a mini split.