A note on the prior comment - I have looked at homewyse and similar cost estimating sites a few times just to see for fun how their estimated cost compared with mine or with actual job costs, or as a cross-check on a quick calc I did inresponse to some question to guard against giving a blatantly wrong number when I was sleepy. Homewyse was generally pretty good, but some of their estimates are WAYYYYY out.
In some cases the reason is obvious - a materials cost that is off by a factor of 10 or set at zero, for instance. In others it was less obvious - I came across one for instance on deck cost, where it read like they were talking about cost of building a deck but their cost actually was applicable to just redecking or changing the actual deck boards only, not building an entire deck, so the cost was off by a factor of over two. I have also seen ones (on that site and others) where the units appeared to be screwed up - maybe it said 100 SF for the example case shown, but the costs were for 1 SF, so the calculated SF price came out to be a miniscule number, off by a factor of say 100. In other cases their materials or labor costs are way off - like $0.10/SF for marble countertop in one case versus $50+, or $10/hour for a plumber. Another one for a geothermal heat pump I remember showed an estimated installed cost less than just the purchase price of a normal gas furnace (without installation) so was low by probably a factor of 5 to 10, so like with everything - even if only using for a ballpark number, check against a number of other sources. I think the most common error I saw was neglecting or grossly under-estimating labor cost for the job - like 200SF per hour for a shower tile layer or only 10% of countertop cost being labor, for instance.
And of course, an architects or engineer's construction cost estimate keyed to your project specifics should be far more accurate, and of course the proof is in the pudding - the actual bids you receive, particularly with the more unusual or exotic jobs. That seems counter-intuitive, but the more difficult or remote or unusual the job, the more opportunity there is for a contractor to either miss the boat and figure a more difficult method and bid high, to underestimate the difficulty or miss some important component and bid low, or to get really innovative and come up with a much cheaper way to do the job.
An example - one very remote site natural resource development job that I worked on, a major series of processing buildings were needed - weighing thousands of tons apiece with equipment. The location was very remote and VERY expensive to get equipment to with zero local workforce, so a major residential camp, fabrication facility, and massive cranes delivered in pieces would have been needed for several years of work to build them on site, at hundreds of millions of $ cost beyond the normal cost for such a facility in a developed area. Working with a specialty mobilization and development contractor, a plan was developed for a set of barge-mounted structures, stick-built on the barges, barged to the site as complete units with provisions for quick-connect utility and walkway and commodity movement connections between the buildings, then moved into a dredged-out canal and inserted in dredged-out spots one by one, the barges ballasted or pumped out as necessary to provide vertical alignment, then hooked up to each other, and finally backfilled around them - leaving the barges in-place as the foundations. Estimated cost savings over $300 million, combining the vastly reduced on-site workforce and camp needs, reduced transportation cost, and vastly cheaper labor by building the barges and facilities overseas.
In another remote site case needing an almost 800 foot deep well to reach suitable quantity and quality of water for a recreational facility in a western state, about $40,000 was saved by recognizing on the maps that there was a nearby abandoned mine ventilation shaft deeper than that, so it was just a matter of putting a well casing and pump down the shaft and installing a quite cheap water treatment unit (which would probably have been needed with a drilled well anyway, in that area).
I have worked on a number of jobs where the low bidder was 25-50% or even as m,uch as 75% in extreme cases below the general bidding range - sometimes due to bidding error or over-optomism, but also sometimes due to them having figured out a faster or cheaper way to do the job. As an owner it can be tempting to either take the low bidder right off based on price, or arbitrarily throw him out as being unrealistically low, but unless you sit down and discuss WHY he was dramatically lower, you might lose the benefit of his special abilities or insight into the job. These sort of benefits appear all the time - packaged versus piece-built HVAC and pump/water treatment systems, concrete-filled foam foundation building systems, pre-manufactured foam-core wall systems, and other types of labor-saving systems.
