Architecture and mechanical systems should be designed together.
HVAC, plumbing, electrical, insulation, water heating, ventilation, networking, solar, batteries, and controls all need real physical space.
Do not finish the architectural concept and then tell the trades to fit everything wherever they can.
HVAC is a perfect example. If no duct routes are planned, the contractor may be forced to run large ducts through an extremely hot attic. The system may still work, but you are making it fight a hostile environment for decades.
Bring HVAC, electrical, plumbing, and structural people into the conversation early enough to influence the drawings.
Architecture has operating costs.
Huge unshaded glass walls, dark roofs, poor insulation, high ceilings, and badly placed ductwork can produce a beautiful house with an ugly electric bill.
The thermal envelope matters: roof, walls, windows, doors, air sealing, insulation, and penetrations.
The roof is part of the mechanical strategy too. Overhangs, roof color, ventilation, and insulation can reduce heat before the air conditioner ever has to remove it.
Water systems need sizing. If several people shower simultaneously, do you have enough pressure, pipe capacity, hot water, and drainage?
Electrical loads are growing with EVs, heat pumps, induction cooking, pools, computers, security, elevators, and batteries.
Future-proof intelligently with panel capacity and conduit.
Technology needs a home too: router, switches, security, automation, audio, and backup power.
Complexity has a cost. Install technology that improves your life, not gadgets that exist mainly for the brochure.
Finally, make every system serviceable.
You are the person who pays the utility bills and repair bills long after the architect and contractor leave.