The answer is "it depends."

Generally, living areas over (unconditioned) garages tend to heat up and cool down more rapidly than 2nd floor living spaces over other conditioned spaces. A separate unit for the space above the garage such as a ductless mini-spilt is typically easily installed and a more economical alternative to zoned, ducted system. If you subtract out the potential living space over the garage, you may find that finishing the remaining areas over conditioned living areas doesn't add much to the required capacity of your HVAC system. I suggest you have your HVAC designer run the heating and cooling load calculations with and without those areas and use that information to inform your decision.

The other issue is - do you want plumbing up in that area. Rough that in now if you do and it is an option. Rough plumbing isn't terribly expensive and can be challenging later. Supply lines aren't too bad but venting and drains can be challenging.

Also, insulate above garage now. If it is unfinished above, then insulation would not be typical (in my area) above the garage. What is typical is drywall so that would need to come down later to insulate. Again cheap thing to do now. Probably want some pics/proof of the insulation for later inspections.

Run electric, gas, refrigerant lines, condensate lines, etc, to where you plan to install the additional system. Then you won't be tearing out drywall, siding, etc, when you eventually install the additional system.

Did you plan to have "700 plus square feet of unfinished space" before the design began? The solution may be to redesign to get rid of it.

How large is the rest of hourse? It helps to know what the ratio of the unfinished versus finished space. What type of HVAC system do you plan to install? Will there be more than one zone? Will the unfinished space eventually be divided into multiple rooms?

You need a competent HVAC contractor who can do a load calculations with and without the unfinished space. For their you can discuss the options. Given what you have described so far, the unfinished space should be a separte zone for better temperature contol. It also gives you the ability to set back the temperature when the space is not being used.

Thanks everyone. Major takeaways as I see it

Plan for rough in plumbing.

Feed for sub panel for future electric needs

Insulate the celling above the garage

Don't plan for a oversize HVAC unit now. Plan for a separate unit or a mini split later

Dont know how to make it as a reply to some one. There was a comment from Mark Bischak, Architect - Did you plan to have "700 plus square feet of unfinished space"

before the design began? The solution may be to redesign to get rid of it.

Thanks but trying to understand a bit more

Let us assume a situation where first floor is 2000 square feet. Based on the current needs, budget etc, necessary rooms are planned in second floor and second floor currently is only at 1100 square feet. . Future plan would be to have rec/game room, gym, additional office room etc

at a later point. With access to these unfinished areas taken care of without going through any bed rooms, what is wrong in this type of design?

@Max G,

Planning to oversize the 2nd floor HVAC system now may indeed be in your best interest, provided the system you install has the required turndown capability. If your 2nd floor living area is 1,100 SF and we assume 350 SF of future 2nd floor living space above conditioned living areas and a 350 SF room over the (unconditioned) garage with a separate, dedicated HVAC system, then the future capacity of the 2nd floor system would be that required for conditioning 1,450 SF. Operating a system sized for that future need at the current 1,100 SF need (assuming uniform heating/cooling throughout the 2nd floor and a separate system for the area over the garage,) would require a turndown capability of (1,450-1,100 SF)/1,450 SF or 25% A two-stage system would provide that turndown, as would a variable-speed system. Have your HVAC designer run the heating/cooling loads for both scenarios and calculate the required system capacities and turndown.

Planning for expansion in the future is good. Designing a house that ends up with extra space that will have a use determined at some future date is not so good.

It sounds like you are planning properly and appropriately addressing HVAC concerns.

As far as design, there is certainly no rule that second floor has to be the same size as 1st floor. You can save a lot of money by not having that extra unfinished space if it is properly designed for. Generally, the extra money to finish that space isn't that much so if you want it, it makes more sense to do at the time of building. Each situation is unique but you will likely pay much more to finish later and you went time without enjoying the space.

With respect to cost-efficiency, a single-story design is typically the least cost-efficient design to construct. A straight-up two-story design is typically the most cost-efficient design to construct (think volume-efficient design.) Deferring finishing some potential living area is a need for many people and a way to increase home equity by investing sweat, the so-called "sweat equity."

Do you have an identified need for this additional space? It sounds like twhat's usually labeled 'bonus room' on developer houses -- where it often exists solely to support a pretty exterior.

If you can't eliminate it, decide how you'll use it, and realize that you'll want to have access to least a toilet and sink for this space. Also consider how your use will affect nearby rooms. (Traffic to and from; noise.) Will it have adequate windows? (Try furnishing it on papaer.)