The unit method of estimating is the simplest and quickest method of estimating the cost of a proposed construction project. In this method, the quantity surveyor counts the number of units which are going to be accommodated in a building, for example the number of occupants or main furniture objects used by an individual. The number is then multiplied by the cost per unit to get the total estimated cost of the project:
Number of Units x Cost per Unit = Total Estimated Cost of Building Project.
As you can see, this method of cost calculation is a rough approximation with a huge margin of error and the quantity surveyor has to account for this margin by giving a cost range rather than a specific figure. In this case, the lowest and highest values are given, including the average.
When to Use the Unit Method?
The Estimator uses this rough estimate in the planning/consultation stage when there are no sketch designs or drawings. On the client’s first visit to an Estimator’s office, the initial discussions are around basic proposals such as the number of people who will occupy the building, the number of rooms/ bedrooms etc. The consultant will then use these numbers to find a rough cost.
A consultant with many years of experience in estimating different types of buildings will be in a better position to give a cost guide of the proposed project. However, the consulting quantity surveyor can also refer to annual building cost data compiled by municipalities and property developers.
In a situation where modular houses are being built, the unit method can come in handy because like a new iphone or gadget that has not yet been released, the cost of a prefabricated home is a factory price that is known in advance. The cost may not include installation but you can easily add an allowance for that.
The unit cost method is best used in buildings where a large percentage of room space is occupied by identical units of furniture or units used by individuals, for example:
- Beds in a hospital – cost per bed
- Seats in a stadium – cost per seat
- Desks in a classroom – cost per desk
- Seats in a movie theatre – cost per seat
- Beds in a hostel – cost per bed
- Beds in a military camp – cost per bed
- Beds in a prison or Cells in a prison – cost per bed
- Computers or cubicles in an internet cafe – cost per cubicle
- Cubicles in a call centre – cost per cubicle
- Workstations in an office – cost per workstation
- Stalls in a market – cost per stall
- Parking Bays in a low-rise or basement park – cost per bay or car
- Seats in a community hall – cost per seat
- Shelves in a supermarket – cost per metre of shelving
Each of these units is measured in cost per unit. The cost per unit is obtained from past projects which were done by the quantity surveyor. If the past project was done more than a year back, the value of the building should be updated.
When the project has progressed to sketch designs, the QS can take off measurements from the plan and prepare an estimate based on the cost per m2 of floor area.
Advantages of Unit Method of Estimating
- It can be done quickly where a ballpark figure is required instantly
- The estimate can be done even if there are no drawings or specifications
- No take-off or measurements needed
- Useful for buildings where standard-sized units occupy most of the space
- A consultant will not charge a fee for this type of estimate.
- Little information is required from client (i.e. number of units), but getting a lot of information from the client will be helpful for the estimator.
Disadvantages of Unit Method of Estimating
- It’s not a reliable method
- Large deviation from the accurate value
- It is not based on drawings or specifications