Thumb rules for caluculating the cost per sqft will work out or not

Yes you can calculate the cost of a building by thumb rule but only for R.C.C, Masonry and internal,external plaster.The cost of finishing items are depends on choice. 

Thumb rules will help to preliminary estimation of any project. for some standard or typical designed works thumb rule is enough to estimate like in mass house construction, govt scheme Indira Avas Yojana, etc.,

It all depends on the experience of the person.  It can give a probable cost as perpared for the preliminary estimates which is required in a short period of time.  While exact calculations are required for the actual and prcise details.

The rule of thumb is a tool most comonly used by Civil Engineers for preliminary discussions only, and never for final approval of estimates. When you are in the middle of discussions with a client regarding the rough cost of his project, rate per Sq. Foot or Sq. M will come in quite handy. But again this rule has to be based on the broad specifications. Making your own rules of thumb comes with experience.

I fully agree with George, it all depend on the requirement, if we want to know the exact cost / budget than we have to go for in depth calculation and that time merly thumb rule will not work out. Thumb rule is required at the initial stage of the project , where we get an idea of lumpsum budget.

So inmy opinion thumb rule will surely work out at initial stage and to get a roughly idea of budget. but it'll not sufficient for accurate cost / budget.

It depends on situation.. For eg. If you want to arrive at an initial budget for simple financial calcuations, Thumb rule for calculating cost/sft is the easiest way. If we need the exact project budget/cost, thumb rule may not help.. detailed drawings and estimate will be required. So it is perfectly requirement based..

Again for a sudden requirement of decision at worksite/project, engineers may depend on Cost based on thumb rules rather than waiting for actual estimate..