Mechanicals are defined as plumbing, heating, air conditioning, and electrical systems. They all have a great deal to do with the comfort of a home and should be taken very seriously.
Both installation and operating costs can vary tremendously. To ensure that you get the most for your money, they should be determined while planning your home, well before construction begins. All of the mechanical trades are regulated by national, and/or state, as well as most local governing agencies. It is very important that your plumber, electrician, and heating, ventilation, and air conditioning (HVAC), specialist be licensed, bonded, and insured.
How they install their respective systems is largely determined by the national and local codes. You must take care to coordinate their activities so as to avoid conflicts, and also to expedite the project.
While you may find a cheaper way to do something, they may refuse, citing building codes as the reason. They have good reason to not violate any codes as they could lose their license. You may want to check the codes yourself. First, find out which code applies to your situation. You can find this out from your local building department. It will usually be a national, and/or state code, plus some local ordinances. A good place to refer to them is at your local library. Do not let tradesmen tell you that some codes are not important and they can do it cheaper. While this may be true in some circumstances, codes are designed for your protection.
The plumbing system has the least flexibility of the mechanical trades. Therefore, it should be installed first, followed by the HVAC system, then the electrical, as it is the most flexible. Switching the schedules of these trades would be asking for trouble and extra expense.
These tradesmen are also a good source of information and advice. Since they are licensed, they should be familiar with all codes, regulations, and special circumstances for your area. As a matter of practice, they will make any necessary calculations to determine the system required for the size and geographic location of your home. You should confer with these trades while still in the design phase of your home.