It depends a lot on the details about your house construction especially the attic and cellar space if there are any and what the wall cavities look like. You can put ducts outside of the house envelope in these spaces. If you can avoid it and run them inside the living space unobtrusively, it is better. Ducts leak. It is not unusual to have 20-30% of the air leaking out of ducts and the all goes away if the ducts are outside. That is really inefficient and causes condensation problems depending on where you live.

Running ducts unobtrusively inside the living space of an existing home is tough to do. People do high velocity systems with smaller ducts. It is a bit of a niche market. Mini splits are becoming more common. I abandoned my ducts last year and had 7 indoor and three outdoor units installed in my home. I had a ridiculous duct run, I like the inherent zoning aspect and their variable compressor and blower speed. I like the fact that I have a lot of redundancy in my system because it is very hot and humid where I live.

My house is raised so the plumbing and cables run under the house and through interior walls, all hidden. Last week I talked with one of my wife's associates who has a "barge board" constructed house. Not to dwell on the details, but they have very narrow wall cavities and no room under the house or in the attic for HVAC ducts or equipment. This is quite different from my situation. They heard I had mini splits and wanted to talk. It sounds like their only real alternative to window units is mini splits. They are tired of their window units and space heaters.

Check out the web sites run by Sanyo, Mitsubishi, Toshiba, and Daikin. Keep in mind that you can go halfway by installing a ducted system where it is advantageous and some mini splits where they might be better.

Where do you live? (What is your climate like?) How do you heat your house now?

"It is not unusual to have 20-30% of the air leaking out of ducts..."

Only with a really shoddy installation.

Losses from heat gain can be significant, especially if you use metal ducts in unconditioned space.

Flex ducts and duct board are better choices in unconditioned space.

Ionized- thank you for your reply. We live in South Florida hence the need for year round AC. This is a house we are looking to buy as it is in a great location close to the beach. The lady that owns it is in her 70's and has done very little updating to it. I believe that house was built in the 40's and of course has the beautiful Spanish architecture common with old homes in South Florida but in the same sense does not have the upgrades for today's living.
The house is built up and has a crawl space underneath. Not sure about the attic....

Keep in mind that II was referring to the existing home stock, not new systems.

I forget, what a reasonable leakage standard for new installations?

That's my kind of climate. I am in South LA. I'll take the old construction, but without the poorly though-out upgrades that are typical, like ducts outside of the living space envelope. If you buy this or some other old house in that area, go to the Building Science Corporation web site and do some reading about what adding air conditioning does to houses that are not designed for it. It can be messy and ugly, mold and rot.

Other resources are your Florida and maybe other the land grant universities in other Gulf coast states. Building and upgrading in hot humid climates should to be done carefully and with knowledge.

How is the house heated now, floor furnace?

We bought a house with in-floor radiant heating so no ductwork. I figured we could have AC installed. The year we bought the house (10 yrs ago) all the bids were $30K to install AC in a 2600 sqr ft house. This was for the ductless mini-splits and mini-velocity systems. We looked at it again about 5 yrs ago. Same high price. We ended up doing it this year (after suffering with a swamp cooler for 10 yrs!) and all in it was about $16.5K and we had to get alot of bids to get the price down to that point. So do check the cost of the AC system to make sure you calculate it in your budget.

So far we are enjoying the cooling capacity of the ductless mini-split. We bought the Fujitsu Halycon system. The only thing I don't like is the bedroom unit. We read not to install a wall unit over the bed as it will blow right on you. So we installed a ceiling cassette. It still blows on us but we can burrow under the covers. However it doesn't matter since it is too noisy to run at night. It says there is a quiet mode but it really is just the lowest fan speed and is much noisier than my air cleaner. I sort of thought it would be similar to the air cleaner but it is loud enough that I can't sleep with it on. There is another unit I could have installed in the attic and ducted to the bedroom. If I could do it again I would do that to keep the noise out of the bedroom and the air from blowing on us while were in bed.

Here is a link that might be useful: Fujitsy Halycon system

For what it is worth, I recently installed a high velocity system in my 95 year old, 2600 square foot house. We have radiator (HW) heat and no ductwork. Moreover, all the walls and ceilings are plaster.

I received 10 quotes--5 for traditional systems with ductwork and 5 for high velocity systems. The traditional systems ranged from $19,000 to $29,000 and the high velocity ranged from $22,000 to $36,000. We did not consider mini-splits after we had a sample brought to the house and placed on the walls. We thought it looked ugly (i.e., out of place) and would not be as good for resale with some people.
In the end, we went for the high velocity system from a very reputable company (installer being far more important than equipment). With a traditional system, we were worried about all the supply and return vents in the plaster. We also were worried about the ductwork runs for the 3rd and 2nd floors due to the attic configuration and length of runs from the air handler. We liked the high velocity system to minimize leakage and its insulation in the longer runs.

The prices above are from New Jersey, so you should be cheaper. Also, efficiency (or lack thereof with high velocity system) was not as important to us because we are using the AC for 60 days maximum in our area. In Florida, efficiency is probably a lot more important because you are using it so much. However, I think one of the experts here would be more qualified to speak to that.

schroads

Wow, it is hard to believe that the Fujitsu ceiling cassette is that loud. My high-wall Mitsubishi are hard to hear even at medium speed. Are you sure it was not working correctly right out of the box?

Note after schrods, you don't have to settle for high wall mini splits. Very small air handlers are available for ducting, as are ceiling cassettes and floor-standing models.