Methods for Obtaining Approximate Estimates

The following methods can be used for obtaining approximate costs of buildings. These are arranged in ascending order of accuracy and computation time : 

• Service unit method 
• Area method 
• Volume method 
• Typical bay method 
• Approximate quantities method 

Service Unit Method 

The most important functional unit of a structure is termed the service unit. A few examples of service unit are given below : 

The cost is obtained from the relation : 

Approximate cost of structure 

= (No. of service units in the structure) x (cost of corresponding service unit in a similar existing structure) 

Note : Current price levels should be taken into account in arriving at the cost of similar existing structure. 

Area Method 

In this method, the cost is obtained as follows : 

Approximate cost of structure 

= (Total area of all floors in the building) x (cost per unit area of a similar exist mg building in the locality) 

The accuracy of the area method can be vitiated by changes in the following factors : Price level, locality of building, ceiling height (as for example in churches, public malls etc.), type of construction and finish, shape of building (rectangular shape less economical than square shape), spacing of columns (owing to changes in structural floor system) and large openings in floors. The Area Method was first proposed for use in obtaining rough estimates for standard buildings like schools, and public housing where ceiling heights are constant. 

In calculating the area of floors, usually what is known as plinth area is computed. Rules for computing plinth area can be referred from "Hand book for Building Engineers, National Buildings Organization, New Delhi",  and these can be utilized for computation. Sometimes an alternative method is used based on 'useful floor area' instead of plinth area especially when computations are cumbersome, as in multi-storeyed buildings. 

This method briefly outlined below : 

Plinth area = Useful floor area + Area occupied by walls. 

This can be rewritten as 

Plinth area = Useful floor area + correction factor (depending on wall area). 

As the thickness of walls increase with the number of storeys, the correction factor increases for lower storeys. Thus, the correction factor can be taken as follows : 

In order to improve accuracy, useful floor area of each floor may be categorised, as follows and relevant rates of unit cost applied to each category : 

1. All functional rooms 
2. Staircase room 
3. Closed verandas or balcony 

Typical rates for unit cost of each of these categories for buildings of similar type have to be ascertained for the estimation. 

The staircase room generally costs 15% more and the closed verandah or balcony costs 40% less than that of the main functional rooms. Again the unit rates are to be applied floor-wise, as intermediate and top floors usually cost less than the ground floor. Usually the unit rates for intermediate and top floors are taken as 80% and 90% respectively of the unit area cost for ground floor. 

The approximate cost of the building = (Useful floor area) x (Correction factor) x (Unit rate for category of rooms of each floor).

Volume Method

In this method, cost per unit volume is taken as the criterion for rough estimate. 

Approximate cost of proposed building = Total cubical contents of buildings x Cost per unit volume of similar structure 

In arriving at the cubical contents, no hard and fast rules are followed, as to 

1. whether the spaces of open verandah, porch, etc. should be taken into account, 
2. how to measure the heights of the buildings, and 
3. whether to take into account the basement and the foundations, and parapets above ceiling. 

The RIBA (Royal Institute of British Architects) rules that are generally followed gives the following procedure : 

Cubical contents = Plan area measured over the external walls x height from top of concrete foundation footing to half-way up the roof if pitched, or to 60 cm above the roof if flat. 

Generally, the volume method can be expected to give a more accurate estimate for single storey buildings than the area method, as it takes into account the ceiling height. But the volume method is very unreliable unless two very similar buildings are compared. For example, comparison of geometrically proportionate buildings can give a very wide fluctuation and lead to an unreal estimate. For, as all the dimensions (size) of the building increase or decrease proportionately, the cubical contents fluctuate far widely than the areas of the component structure. Also the volume method does not take into account the number of floors or storeys. Also, such items, as lifts are generally, included as lump-sum, since their costs do not bear any relation to their volume. 

Typical Bay Method

This method is useful for application to framed structures consisting of a number of similar bays, as for example, industrial buildings. A typical interior bay (between two successive columns) is selected and its total cost worked out. 

Then, the approximate cost of building = Number of bays in the building x cost of one bay. 

Suitable correction factors may have to be used to take into account the dissimilarities in end-bay (and in different floors, if applied to multi storeyed construction). 

Approximate Quantities Method 

In this method, the main items are taken into account using short-cuts in the approximate calculation of quantities of main items such as walls, roofs, doors and windows, and finishes, and the cost is computed by adopting current unit costs for these items. The grouping of items and fixing their approximate cost requires good judgement and experience. This method though time-consuming, is more accurate than any of the above approximate methods.