14-02-2013, 02:03 PM
Estimate Types
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Construction estimators use four basic types of estimates. These types may be
referred to by different names and may not be recognized by all as definitive,
but most estimators will agree that each type has its place in the construction
estimating process. The type of estimate performed is related to the amount
of design information available. As the project proceeds through the various
stages of design (from schematic design to design development to contract
documents), the type of estimate changes and the accuracy of the estimate
increases. Figure 1. 1 graphically demonstrates the relationship of required time
versus resulting accuracy for these four basic estimate types.
1. Order of Magnitude Estimate: The order of magnitude estimate could
be loosely described as an educated guess. It is also known as a
“napkin estimate,” because it is often the result of a conversation
between a contractor (or developer) and a client/owner over lunch or
dinner, in which an estimate is created on the nearest piece of paper,
usually a napkin. Order of Magnitude Estimates can be completed in a
matter of minutes. Accuracy is plus or minus 20%.
2. Square Foot and Cubic Foot Estimates: This type of estimate is most
often useful when only the proposed size and use of a planned building
is known. Very little information is required. Performing a breakout
for this type of estimate enables the designer and estimator to adjust
components for the proposed use of the structure (hospital, factory,
school, apartments), type of foundation (slab on grade, spread footing,
piles), and superstructure (steel, concrete, or a combination) and to focus
the cost more closely to the final price. Accuracy of the square foot
estimate is plus or minus 15%.
3. Assemblies (or Systems) Estimate: A systems estimate is best used as a
budgetary tool in the planning stages of a project. Accuracy is expected at
plus or minus 10%.
4. Unit Price Estimate: Working drawings and full specifications are
required to complete a unit price estimate. It is the most accurate of the
four types but is also the most time-consuming. Used primarily for
bidding purposes, accuracy is plus or minus 5%.
Unit Price Estimating Methods - Copyright 2009 - RSMeans/Reed Construction Data
Order of Magnitude Estimates
The Order of Magnitude Estimate can be completed with only a minimum of
information. The proposed use and size of the planned structure should be
known and may be the only requirement. The “units” to describe the structure
can be very general, and need not be well-defined. For example: “An office
building for a small service company in a suburban industrial park will cost
about $650,000.” This type of statement (or estimate) can be made after a few
minutes of thought used to draw upon experience and to make comparisons
with similar projects from the past. While this rough figure might be
appropriate for a project in one region of the country, an adjustment may be
required for a change of location and for cost changes over time (price
changes, inflation, etc.).
Figure 1.2, from Means Building Construction Cost Data, shows examples of a
different approach to the Order of Magnitude Estimate. This format is based
on unit of use. Please note at the bottom of the categories “Hospitals” and
“Housing” that costs are given “per bed or person,” “per rental unit,” and “per
apartment.” This data does not require that details of the proposed project be
known in order to determine rough costs; the only required information is the
intended use of the building and its approximate size. What is lacking in
accuracy (plus or minus 20%) is more than compensated by the minimal time
required to complete the Order of Magnitude Estimate—a matter of minutes.
Square Foot and Cubic Foot Estimates
The use of Square Foot and Cubic Foot Estimates is most appropriate prior to
the preparation of plans or preliminary drawings, when budgetary parameters
are being analyzed and established. Please refer again to Figure 1.2 and note
that costs for each type of project are presented first as “Total project costs”
by square foot and by cubic foot. These costs are then broken down into
different construction components, and then into the relationship of each
component to the project as a whole, in terms of costs per square foot. This
breakdown enables the designer, planner, or estimator to adjust certain
components according to the unique requirements of the proposed project.
Historical data for square foot costs of new construction are plentiful (see
Means Building Construction Cost Data, Division 17). However, the best source of
Order of Magnitude
Estimates
Square Foot and
Cubic Foot
Estimates
Unit Price Estimating Methods - Copyright 2009 - RSMeans/Reed Construction Data
square foot costs is the estimator’s own cost records for similar projects,
adjusted to the parameters of the project in question. While helpful for
preparing preliminary budgets, Square Foot and Cubic Foot Estimates can also
be useful as checks against other, more detailed estimates. While slightly more
time is required than with Order of Magnitude Estimates, a greater accuracy
(plus or minus 15%) is achieved due to more specific definition of the project.
A Square Foot Estimate is consistent with the amount of design information
available at the schematic design phase.
Rising design and construction costs in recent years have made budgeting and
cost efficiency increasingly important in the early stages of building projects.
Never before has the estimating process had such a crucial role in the initial
planning. Unit Price Estimating, because of the time and detailed information
required, is not suited as a budgetary or planning tool. A faster and more costeffective
method is needed for the planning phase of a building project; this is
the “Assemblies,” or “Systems Estimate.” An Assemblies Estimate is usually
prepared when the architect completes the design development plans.
The Systems method is a logical, sequential approach which reflects how a
building is constructed. Seven “UNIFORMAT II” major groups organize
building construction into major components that can be used in Assemblies
Estimates. These UNIFORMAT II divisions are listed below: