What appears to be a bona fide "first" in virtual realty is a consumer Web site devoted to estimating the cost of building a new home or rebuilding your existing one.

Still missing in cyberspace is a Web site that lets you estimate the cost of individual remodeling jobs, but if some construction calculator wizard comes up with such a site, it ought to model Building-Cost.net's new home building cost estimator.

The estimator is a handy tool for both actual building plans or to help determine your insurance policy's home replacement value -- provided you underscore and understand "estimate".

The welcomed site also offers educational and explanatory content, all to often missing from such digital number crunchers.

Produced by construction book and software publisher, Carlsbad, CA-based Craftsman Book Co., the Net-based home building cost estimator uses data from the publisher's "National Building Cost Estimator" (Craftsman, $23) book.

You'll also need an intimate knowledge of your planned or existing home's building materials, style, square footage and the like, but in a few minutes you'll get an estimate of what it will cost you to build -- or rebuild -- your dream home.

Plug in your planned floor plan, square footage, quality of structural components from the foundation to the rafters, additional living spaces, urban or suburban location, heating and cooling type, type and number of fireplaces and location (by ZIP code group/city) and the calculator cranks out estimates for material, labor and equipment in 34 construction categories.

It's not as simple as it sounds, but what helps this calculator add up is Craftsman devoting an entire Web site to the process. That means, rather than just blankly filling in the blanks, there's plenty of rich content to guide you.

At each step of the estimating process, you get instructional content to help you determine your most accurate answer. For example, determining the level of quality you'll use requires that you select from among four levels of quality for each of 10 different components, including the foundation, plumbing and roofing. The site offers concise information about each quality level, for each of the 10 areas.

Craftsman concedes both its manual and the Web site are reference materials. Even the most competent estimators with blueprints, on-site inspections and current labor and material costs often disagree on the cost of a building.

"Frequently, exhaustive estimates, for even relatively simple structures, can vary 5 percent or more. The range of competitive bids on some building projects is as much as 10 percent. Estimating costs is not an exact science and there is room for legitimate disagreement on what the "right" cost is," says a disclaimer on the publisher's parent Web site.

Given the cost differences and volatile pricing shifts in micro markets, estimates from Building-Cost.net's consumer calculator could miss the mark by a greater margin.

But that same general "for-reference-only" disclaimer applies to the more sophisticated Axion/DataQuick-Marshall & Swift Home Replacement Cost Estimator available only to fee-paying professionals, as well as to more specialized on-line building calculators that let you zero in on the cost of materials alone.

It's impossible -- today -- to replace an informed professional working in the field with an online calculator accessed from your computer.

Building-Cost.net is instead a solid "getting-started" or "Can-I-afford?" type estimator.

Craftsman, which also publishes, the "National Repair and Remodeling Estimator" (Craftsman, $48.50), is also the best bet to come up with a similar site for estimating the cost of individual home improvements.