2nd floor plan. Red marks existing supply, green marks our ideas for new supply. Boxes with an "R" are registers. Boxes with an "X" are existing registers we could seal off, if we add new ducts.

I think you'll have a more predictable--and better-- outcome if you have a professional run the heating/cooling load calculations and design the ductwork to be retrofitted. It's important to know whether your existing HVAC system has sufficient capacity to add the additional air flows and that it will actually supply them to the 2nd floor once the additional duct work is installed.

Yes agree. As I said, we absolutely plan to have a professional do a real design. Just looking for rough ideas about method and cost, to figure out whether to pursue this now. Even getting an estimate right now takes months of waiting. If it's a $10k job, we will get in line and try to get a professional out within a calendar year. If it's a $40k job, we will just buy 4 window units and look for a contractor in 5 years.

It might be more effective and easier to add a minisplit with a head in each room to cool the upstairs spaces. Cost should be in the $10K range, but you need a local estimate. If the rooms are not in regular use it gives you some energy efficiency as well, as each room can be controlled independently.

What does a retrofitted AC mean exactly? Have you determined if the size of the equipment is large enough to cool the whole house? You need to do this first before spending a lot of money trying to open up walls and ceilings to install new supply lines.

A mini split is a good option for adding AC to a house built in the 1950s. I suggest you look into getting some quotes.

By "retrofit" I mean that previous owners hooked up an AC unit to the 1958 ductwork, which isn't designed to deliver AC. It's a 1750 square foot house, 3-ton AC unit, Seattle area. So pretty sure the equipment is the right size for the house.

I got a quote in 2017 for minisplits. It was $10k then. So probably $15k now. Enough that we want to investigate improving the current system before going that route.

This type of work is unique to your house and is difficult for anyone to give an accurate estimate of the cost. Your best option is to get 2-3 quotes.

Thanks to all. I'll call some people then and see if anyone will come give us estimates sometime in the next calendar year.

You would be better off replacing and modernizing both heating and cooling. 1958 ductwork probably contains asbestos, and isn’t properly insulated.

We have had a MrCool DIY minisplit for 9 years now with no problems. We are in a moderate climate where it heats and cools (temp. range 0-88 F with high humidity)

These units come fully charged and are relatively easy to install. The 12,000 btu will condition about 500 square feet and cost is about $1200 now I believe.

Of course tools are needed to drill a hole for the lines to the indoor head, and an electrical hookup plus an exterior placement for the unit, etc. But very affordable in comparison.

Ductless mini splits will be much easier to install on a retrofit basis and your job will be easier to fit into a contractor's schedule. They'll enable independent temperature control of the 2nd floor which is a big plus.