I'm looking at maybe building a house soon and had a few questions regarding the central ac unit,1st question is how to figure was size unit I need,I can't find any good info on the internet,2nd is how much should I look at paying for the unit and installation including duct work and whatever else is required, 3rd is what's the lowest SEER rating I should go with and what brands are the best,my house will be 2800sq/ft,I've got one estimate but just trying to see if these folks are putting the screws to me or not,any info from any AC guys or contractors that build houses would be appreciated

I be willing to bet that they go by sqft. If I were building a home that was two stories I would end up having two smaller units. one for the top floor and the other for lower floor.

Shoot, I'd probably even look into PTAC units for like the master bedroom and man cave area just to cut down on cooling the whole house instead of just the area you will be in.

We have a two story house now and instead of turning the ac down at night and cooling the whole house I put a window unit in the master bedroom and use it at night turning the temp up on the central air.

I'd also worry about the heating, learned last winter that having a good fireplace/woodstove would have saved us hundreds in energy bills.

A general rule of thumb is a ton per 600sf.

I know they go by sq footage but does ceiling height,# of windows,and different things like that play into it?dont want an oversized unit and dang sure don't want it undersized,its a one story house and the bonus room will have one of the small Mitsubishi type a/c units.the 2800 sq ft doesn't include the bonus room

^^This is a pretty good rule of thumb. The company I work for has a climate division (fancy way of saying heating/cooling division) and they push one ton for every 500 sf.

you will need to refrence the Manual J book to calculate the heat load on a house

http://www.loadcalc.net

A ton per 500 sf is usually okay for our area Gobblebox. And yes all of the above and more affects the HVAC design/sizing. The ductwork design is at least as important as the unit. If the ductwork, vents, returns, etc are incorrectly sized, you won't be happy. if you want, pm me who you are using and I'll give you my two cents on them . I assume you are building here locally.

Typically, it's about 600 sq ft per ton on 8-9 foot ceilings, about 550 on 10-11 ft, and 500 Sq ft per ton on 12-14 ft ceilings. At 2800, you should look at two zones. I'd do the bedrooms on one and living area on the other with programmable thermistats. That'd what I have and works great. Mine is 12ft ceilings and I have a 3 ton on the 4 bedrooms and 2 baths, then a 3.5 ton on the rest downstairs. Bedroom and bath upstairs is about 300 sq ft and has a 1 ton unit up there.

If you can afford it look at the Mitsubishi ducted systems.
You can have multiple zones in the entire house and use one outdoor unit if the load is under 48,000 BTUs. The amount of money you save pays you back multiple times over.
Make sure you get someone to do a load calculation.
Alabama Power used to offer that service and may can help, not sure about TVA.
Good Insulation pays you back EVERY month!