HVAC companies are usually able to quote based on the plans you've supplied, and sometimes give input on locations and how to improve the design. A lot don't want to really assist in the designing stage until you've signed a contract with them and they know how much $$ are coming back into their pockets. I suspect they rather like seeing plans that don't work well for heat distribution and then they get to jack the price if you don't want bulkheads and you don't want to change your plans.

Experienced architects will provide location space for HVAC equipment, and if you have them do detailed interior plans, elevations and sections, will locate supply and return diffusers, controls, etc., taking into account structural framing.

An architect's specifications typically contain the "performance" criteria to be used by the HVAC contractor (if you pay the architect for work beyond simple floor plans).

The actual design of the HVAC system is typically done by the HVAC subcontractor selected by the builder and the cost is included in the builder's bid.

You should be aware that once construction has begun, it's always somewhat of a contest between mechanical, plumbing, electrical, signal and other system subs as to the sequence of trades called to the job site by the builder. Whomever is first tends to run their system without much regard for subsequent subs. Subsequent subs wind up "fitting around" the work of earlier subs. This may not pose a problem in "simple" houses, but can create challenges in more complex designs.

Good luck on your project.

Thank you Logbuild and VirgilC.

We were concerned that we might need a larger area for HVC. We left a space open for a larger run to the second floor, but we're not sure if its needed.

I differ with some of the previous comments. For HVAC, Manual J and Manual D calculations can be done just from the plans. That plans the size of the equipment and basically how and where the registers will be placed and the duct sizes supplying those registers. This function is now mostly done with excellent computer programs.
But the sequencing of the trades is not negotiable, in my opinion. A good builder will know how it should be done-- lots of builders are not good ones.
The plumber must have first choice for installing the DWV system -- sewage doesn't flow uphill!. A good plumber will try to make the DWV system as unobtrusive as possible -- but proper functioning is the prime consideration. The plumber has considerable flexibility with the Supply piping. Then the HVAC gets next dibs on the space available. The poor electrician gets whatever space is left after that.
Any other sequence is likely to have poor results.

In the case of my present residence, the only house i have built from scratch, I brought in the HVAC contractor when the basement floor was just gravel to plan for the condensate drain in the floor. My preferred location for the HVAC air handler to be tucked away to one side was doable, but at higher cost and with much more large ductwork to get proper distribution. Ended up locating it for the simplest ducting system and placed the condensate drain in the floor there. No regrets.

There are a number of early design decisions which need to be made about HVAC systems, if the final choice of a system is to meet the owner's expectations, be integrated into the project in as compatible a manner as possible, and operate with the desired efficiency.
For example, what energy sources are available for the HVAC system? What are the climatic conditions in which the HVAC will have to operate? What is the energy strategy and efficiency desired for the house envelope and the HVAC system and distribution/return? What is the budget? What is the preferred relationship between installation cost and operating expense? Where and how will the HVAC system be placed and how will distribution and return be handled? What sort of zones, controls and other flexibility is desired? And so on...

Once these early design decisions are made, running the calculations is simple enough, especially if one has a dedicated computer program for the purpose. Before being able to run the calculations, however, is where all of the influential decisions are made.

Good luck on your project.

Thank you BusDriver for your thoughts. I'm not as much worried about which trades come when, I hope the GC will handle that, as much as I'm considering how HVAC will affect the overall design.

Thank you VirgilCarter. I think I understand your comment to mean that being fully aware of the various constraints on a project will be beneficial in determining the HVAC system's design. If that's a correct understanding, I agree!

Thanks!

I would definitely get a Manual J (load calculation) at a minimum to spec out the proper size hardware. To get one though, you need all sorts of specs including r-values of the insulation, shgc and u values for the windows, etc. I would get this when you are closer to building. Also, if you hire your own engineer to do the system, they probably need some input from the hvac contractor. They will need to know the common size ducting that they use, r-value on the ducting, rigid or flex duct, etc.

I personally would use an outside place or make sure your hvac shows you the numbers. As you say...too many people dont want to plan correctly and they use a "rule of thumb"