If your climate is relatively mild, maybe even not so mild, the mini split might be fine for heating and cooling and save you money.

Agree with above answer.

How regularly will the space be used? Do you want the end result to be upscale in appearance and function, or is just "adequate" and less expensive okay/desired?

Where is the noise minimization important, inside or outside or both?

400 sq.ft. is too small to install even the smallest central unit, even though there are a few manufactors that still make a 1 ton central unit. As stated above, It sounds perfect for a heat pump mini split. There are alot of them on the market, ranging from inexpensive to very expensive. If you want it dead silent inside the room, then i would suggest a unit with the vari-spd.,(frequency modulated)compressor. The indoor units fan motor is also vari-spd. and they can ramp down to where they are almost silent. If noise is not an issue, any of your standard mini splits should be fine. Indoor units are by no mean noisy but they are a little louder than the higher end units. Of course the vari-spd. units have a higher seer rating than the single spd. units do, some up to 22 seer but get ready to pay alot more. We have been installing alot of Thermal Zone units as our standard units and Fujitsu as our higher end units. Extremely happy with both of them.

Hope this helps

Thanks to all for the input. It sounds like a mini-split is the way to go.

Our climate is relatively mild (northern CA -- an occasional sub-freezing day in the winter and maybe a week breaking 100 in the summer). The space will initially be an occasional use space (houseguests), but it's also being built in anticipation of one/both of my inlaws living there when it comes time they need more assistance.

I'd really like to minimize the noise inside -- I think we should be able to manage exterior compressor noise as the building will be at the back of our lot.

I guess my initial trepidation of a mini-split was concern it might be challenged with the volume/height of the space (open 15 ft ceilings). I was thinking radiant for heating given the slab construction and our (asian) parents' no-shoes living style.

It sounds like those fears might be unfounded?

Thanks again!

As Ionized says, the outdoor unit of the minisplits is actually probably the quietest "compressor" you could get. I know one person who specifically chose minis because he has tiny setbacks and really tight noise regs (near/on the water). He is THRILLED!

Mine are right outside (I have 2) my dining room bay windows, and you don't really hear them sitting right next to the windows (I have "leaky aluminum windows there still too--need to get them taken care of).

If you're in the PG+E service area (as I am) and this living space isn't separately metered, you might find that using electrical heat (a mini-split unit) is VERY expensive. Any natural gas solution, whether a radiant slab or a wall mounted heater unit with a fan, will be much less expensive to operate. With either of these, you'd also build in a through the wall A/C unit for summer cooling.

So I've been thrown a curveball since my original post.

Our furnace for the (main) house just went kaput (igniter problem which, upon inspection for repair, revealed a cracked heat exchanger). So I need to get some sort of replacement in the next week.

The guest studio on which I originally posted was to be Phase I of a two stage complete rebuild of our house -- Phase II was going to be a rebuilding of the main house with a considerable expansion (current house is 1200 sf, single floor bungalow -- future home will be close to 3000 sf split across a partial daylight basement and split level aboveground structure).

We had originally planned/hoped the existing home mechanicals would survive until Phase II (probably 1-2 years off), so today's discovery puts a little bit of a kink in our plans.

The HVAC guy I had come in to look at our furnace is going to see if they have any suitable used furnaces for a cost-effective replacement (anticipating it will have only a 1-2 service life needed for us), but he also thinks we might be able to install a new, high-efficiency furnace and then re-use it either for the guest studio or for one of the floors (e.g., the basement level will be about 1000sf) in the updated main house.

I'm a little leery of planning on reuse since there are so many unknowns this far out from actually breaking ground and imagine that a number of things could come up that would make re-using the unit impractical once the time came, but I would be interested in what the folks on this forum think.

I might be less worried about re-using the unit in the guest studio (as that timing is closer in), but am concerned on the size/capacity compromises we might be making (1200 sf house to a 450 sf guest studio).

Cheers,

Ed