Construction Lengths For Commercial And Residential?

Usually the time difference for comparable sq. footage is based on a couple of reasons. 1.Custom work, meaning one of a kind shapes and sizes, if you know you are going to need ten thousand studs cut 92 5/8″ you can purchase them already cut, by the same token if each stud needs to be 1/4″ longer you are going to have to cut each individually. This happens with each phase of construction, not just the basic structure. Electrician will have long and short and odd runs. same for all the mechanicals. 2. Repetativeness, if someone does something over and over they are going to get faster at it or find little shortcuts. Whereas in the custom house just about the time you figure out hte shortcut you are done with the job. 3. Craftsmanship, on custom houses there is usually a much higher standard to adhere to, both quality of work and materials. The higher quality materials take more skill and labor to install. ex. a stone wall as opposed to a cement block wall. These are just a few of the things,the list is a lot bigger but I hope this gets the idea across

The simple answer is the completion time specified by the owner. A mansion owner would be more inclined to want it built right, than how quickly it is completed. An office building owner would want it done quickly, because the sooner it is built, the sooner it can make money.
Some government buildings are built in 18 months total, including the design, which usually takes 6 months.