Estimating cost of construction methods

An accurate reliable estimate or forecast of the probable cost of construction of any building or civil engineering work can only be found out after planning, designing and preparation of detailed drawings. The process will involve working out and pricing detailed quantities for each item of work like excavation, concrete, brickwork etc., applying current market rates and then arriving at the total cost. This entails considerable effort and expenses, and before undertaking the same a becomes necessary to assess the likely cost of construction using approximate methods.
Approximate estimates may be defined as preliminary forecasts indicating the likely cost of construction. Approximate estimates are used for :


  • making administrative/financial decisions about talking up the work in hand, so that detailed planning can be initiated after getting budgetary approvals,


  •  assessing commercial viability, i.e. working out probable net returns on investment,


  • carrying out building economics/cost planning exercises to decide on the shape/size/construction methods and the like, to be adopted at the time of detailed planning, and for studying aspects like taxation, insurance etc.

Plinth Area or Square Meter Method

In this method the built-up area measured at each floor including basement is worked out and added up. Approximate cost of the proposed building is then worked out by multiplying the total of plinth area of all floors by construction cost per unit area of a similar building built in the recent past. The cost per unit area adopted in the calculation should be suitably modified to cater for the following variations between the building from which the rate is adopted and the building for which the approximate estimate is required : difference in ceiling height and plinth height; difference in type of construction or specifications; appreciable difference in location; difference in date of construction; change if any, in the price level of important items or work; and the like.


Cubic Rate or Cubic Meter Method.

This method is based on the volume of a building in cubic meters, inclusive of hollow spaces inside the rooms etc. The volume of a building inferred in this method should not be confused with the volume of solid content of the building like brickwork, concrete etc. Approximate cost of the proposed building will be volume in cubic meters of the proposed building multiplied by the cost per cubic meter of volume of a similar exisiting structure built in the recent past.
For working out volume of a building in cubic meters, built-up area of the building at each floor level is multiplied by height of the respective story, except that the height of ground floor is measured from half the depth of foundations, and height of top floor in case of pitched roof is measured upto half the height of slanting portion of the roof. There is however no uniformity of rules for calculating height of the building, inclusion or exclusion of open narrow ventilating shafts, roofed over galleries/verandas etc., for the purpose of calculation volume of the buildings. This diversity of rules will however not make any difference as long as calculation of volume of the building from which the rate per cubic meter is derived is done in the same manner as calculation of volume of the building for which the approximate estimate is to be worked out.
Modifications required in the rate per cubic meter to allow for variation between the two buildings being compared will be same as the factors mentioned in the plinth area or square meter method.


Service Unit Method.

The term 'service unit' relates to the purpose of a building or civil engineering work. Approximate estimates for the following types of buildings/works can be made using the service unit method :

Building/work   
Service unit
   Building/work   
Service unit  
Building/work    
Service unit
College/school
Class room
     Stable
   Animal
Highway
  Kilometer
Hospital
Bed
     Hotel
   Room
Canal
  Kilometer
Cinema/stadium
Seat
     Hostel
   Student
Water/sewage main
  Meter
Prison
Cell
  Large apartments
   Tenement
Water tank
  liter


The approximate estimate in this method is based on the cost per service unit, incurred for a building or work constructed in the recent past. Necessary modification to the unit rate will have to be made to cater for the variations described in the plinth area or square meter method given above.
Example : Cost of constructing a hostel building to accomodate 100 students (two per room) including all furniture, toilet blocks, verandah etc. was Rs. 30,20,000. This yeilds a unit rate of Rs. 30,200 per student for similar hostels.



Typical Bay Method.



Typical bay method for working out approximate estimates is useful for buildings like row of garages, factory, buildings, railway platforms etc., which have several identical bays. A bay is a typical compartment of a building. For example, a typical bay of a row of garages will be portion of the building from center to center of two successive columns (or cross walls) in one direction, and from outer face of front wall upto rear face of rear wall of the bay in the other direction. The end bay will need to be considered separately and appropriate adjustment made to cater for gable wall etc. The approximate estimate will be number of intermediate bays multiplied by cost of one intermediate bay plus the cost of the two end bays.




Approximate Quantities Method.



In the approximate quantities method all-in-price of each different element or  component of the building is worked accurately  as possible as the first step. For example, (a) all  costs involved in the wall per meter length upto plinth including foundations, damp proof course etc., (b) cost of wall in superstructure per square meter including brickwork, plastering, distempering etc., making a prejudged adjustment for openings etc worked out as a percentage of the measured all area., (c) cost of RCC slab per square meter inclusive of concrete, reinforcement, centering, formwork, plaster/whitewash its soffit etc., and all such principle components of the building. The number of units of each element or component are worked out approximately and multiplied by the all-in-price per unit. All these costs of the different elements of the building item added up will give the approximate cost of the building, after making an allowance for sundry items overlooked.




Empirical Formulae Method.



Approximate method of estimating cost of a building based on empirical formulae worked out by C.B.R.I. Roorkee labour and material requirements of different types of buildings. The qunatities materials and labour days multiplied by the current market rates for the same, yeild the approximate construction cost the building when added up.



Building Cost Index Method.



The building cost index method of approximately estimating the cost of a building is suitable for Government departments like CPWD/MES/Railways etc. whose construction activity continues year after year, and is spread all over the entry. Average cost of construction of a particular type of building, in usually the head-quarter region of the department, particular year is worked out. This worked out cost, for the particular type of building is allotted the index 1 (one), the particular year is treated as the base year. As construction costs will increase with passage of time, the building index after a passage of say five years may be 1.25 in relation to the base year for the particular region. Similarly, as construction costs vary from one region to another. the construction cost index for a particular region can be worked out relation to the index of the head-quarter region. Thus the variation in price due to passage of time as well as regional reference in construction costs can be monitored, and effectively used for preparing approximate estimates.
Head offices of the various departments receive feed-backs from their zonal or regional offices intimating the construction actually incurred on recently completed buildings/projects, and this information is comiled and disseminated to all informed with preparing approximate estimates for the department.