Types of Estimate

Meaning

An approximate (preliminary) estimate is a rough estimate of the probable cost of a project, prepared quickly from the general requirements and past experience, without detailed drawings or item quantities.

When and why it is prepared

1. At the planning stage, to decide whether the project is feasible and affordable.
2. To obtain administrative approval and budget allocation from the ministry/municipality.
3. To compare alternatives (sites, designs, types of structure).
4. For funding and loan applications, and in the preparation of the annual programme.
5. When there is no time or data for detailed estimation, as at the start of a design.

Short note

• Methods: plinth area method, cubic content (cube) rate method, unit base (per bed, per student, per km, per cum of storage) method, approximate quantity method, and the cost of a similar building in the locality.
• Data: area of the building, the prevailing plinth area rate, percentages for services and contingency.
• Contents: cost of the civil works (plinth area x rate), special architectural treatment, water supply and sanitary, electrical works, external services, contingency and supervision charge.
• Accuracy: about 10 to 15% from the final cost; it is not used for tendering.
• It is followed by the detailed estimate after design is completed.

Circumstances (summary)

• Preliminary: the approval stage when drawings are not ready.
• Detailed: once drawings are available, before tendering; it is the basis of the contract.
• Revised: when the original sanction is exceeded; it replaces the old estimate.
• Supplementary: a new addition to a sanctioned work; the original remains valid.
• Complete: where the project cost includes more than construction.

Contingencies

Contingency is a lump-sum allowance (usually 3 to 5% of the estimated cost) added to the estimate to cover small, unforeseen expenses that cannot be foreseen at the time of estimating: small changes in quantity, minor extra items, errors and omissions, and temporary rise in costs. It is spent only with the sanction of the competent authority. It includes also items such as the cost of stationery, travelling and testing in some practice.

Work-charged establishment

Work-charged establishment means the temporary staff engaged for a specific work and paid from the cost of that work rather than from the regular office budget. These include supervisors, overseers, store keepers, watchmen (chowkidar), chainmen, mates and clerks employed for the project period. An allowance of about 1.5 to 2.5% of the estimated cost is added to the estimate for it. Such staff are released when the work ends.

A revised estimate is a fresh estimate prepared to replace the original sanctioned estimate when it is found that the original will be exceeded.

Why it is prepared

• To bring the sanctioned amount in line with the actual probable cost, so that payment beyond the sanction does not occur.
• To obtain approval for the changed cost before further expenditure.

When it is prepared

1. The cost is likely to exceed the sanctioned estimate by more than the permitted limit (about 5%, as per the rule of the agency).
2. Changes in rates of material or labour, or a new District Rate.
3. Design changes or changes in the specification required by the site.
4. Major variation in quantities, such as deeper or rock foundation.
5. Cost of the work already done and the balance work have to be re-estimated after a long delay.

Example

A school building was sanctioned for Rs 20,000,000. During excavation, the soil was found weak and the foundation had to be deepened, adding Rs 2,000,000 (10%). As the increase exceeds the limit, a revised estimate of Rs 22,000,000 is prepared, giving the original items, extra items and the reason for each change, and sent for fresh approval.

A preliminary estimate is prepared by the plinth area method, as follows. It is for the administrative approval, so the drawings are not yet detailed.

Steps

1. Establish the requirement: the number of staff and rooms, and from these the total carpet area (usable floor area) from the norms for office space per person.
2. Find the plinth (built-up) area:

$$\text{Plinth area} = \frac{\text{Carpet area}}{1 - (\text{circulation \%} + \text{wall \%})}$$

The circulation area (corridors, verandah, stairs, toilets) is taken as 20 to 30% and walls as 8 to 10% of the plinth area. 3. Select the plinth area rate prevailing in the locality (or the DUDBC / department rate) for the type of building. 4. Building cost = plinth area x plinth area rate. 5. Add the extras as percentages of the building cost: special architectural treatment, water supply and sanitary works, electrical installation (and lift), deep foundation, and other services. 6. Add external services: compound wall, approach road, drainage, septic tank and landscaping. 7. Add contingency (3 to 5%) and supervision/departmental charge (3 to 8%). 8. Total is the estimate for approval.

Example (assumed)

Carpet area 1,000 m²; circulation 30% and walls 10%, so plinth area = 1000 / 0.6 = 1,666.67 m². Plinth area rate Rs 20,000 per m² gives a building cost of Rs 33.33 million. Electrical at 8% and other services at 5% add Rs 4.33 million. Contingency 5% and supervision 5% are then added on the total. The result, about Rs 41.4 million, is submitted for approval.

Method

Plinth (built-up) area is found from the carpet area, as the carpet area is what remains after the circulation (30%) and wall (10%) areas are removed:

$$\text{Plinth area} = \frac{2000}{1 - 0.30 - 0.10} = \frac{2000}{0.60} = 3333.33\ \text{m}^2$$

This is the total plinth area of all 4 floors (833.33 m² per floor).

Calculation

Answer: Plinth area = 3333.33 m²; total estimated cost = Rs 62,975,000 (about Rs 6.30 crore).

Contingency and supervision are taken on the total building cost (Rs 57,250,000) as given in the question.

A complete estimate gives the total cost of a project, covering every cost needed to make it ready for use: land, building (civil) works, services, furniture and equipment, design and supervision, contingency and taxes.

Why it is prepared

• To know the full financial commitment, not only the construction cost, so the owner or donor can sanction the money.
• To avoid supplementary demands later, because nothing important is left out.
• To compare the cost of the project with its benefit and to arrange the finance.

When it is prepared

• At the project planning and approval stage, after the detailed estimates of the individual works are ready.
• For large projects such as hospitals, hostels, dams and water supply schemes where land, equipment and external works are included.
• When a loan or a grant is requested.

Types of estimates

1. Preliminary (approximate) estimate
2. Detailed estimate
3. Revised estimate
4. Supplementary estimate
5. Complete estimate
6. Annual repair and maintenance estimate
7. Extension and improvement estimate

Revised versus supplementary estimate

The accuracy is about 10 to 15%, best for the cube rate method, and the least for the unit base method. The estimate must be adjusted for the location, the year and the specification.

Plinth area

Carpet area is what remains after circulation (20%) and walls (7%) are taken from the plinth area:

$$\text{Plinth area} = \frac{3275}{1 - 0.20 - 0.07} = \frac{3275}{0.73} = 4,486.30\ \text{m}^2$$

Estimate

Plinth area rate = Rs 25,000 per m² (the paper prints m³, taken as m² because the rate is applied on the plinth area). Extras are taken on the civil cost; contingency and supervision on the total building cost, as the question says.

Answer: Plinth area = 4,486.30 m²; total estimated cost = Rs 156,241,053.08.

Assumptions

The total carpet area of 500 m² is for the whole four-storey building, and the plinth area is the same on each floor. Contingency is taken as 5% and work-charged establishment as 2.5% of the total building cost.

Plinth area and volume

$$\text{Total plinth area} = \frac{500}{1 - 0.08 - 0.10} = 609.76\ \text{m}^2$$

Plinth area per floor = 609.76 / 4 = 152.44 m².

Height of the building = 4 x 3.0 + 1.2 (parapet) = 13.2 m.

$$\text{Volume} = 152.44 \times 13.2 = 2,012.20\ \text{m}^3$$

Estimate

Answer: Volume = 2,012.20 m³; civil cost = Rs 10,060,975.61; total estimated cost = Rs 14,060,213.41.

Carpet and plinth area

Carpet area of classes = 600 x 1.2 = 720 m²; add offices 50 m²; total carpet area = 770.00 m².

$$\text{Plinth area} = \frac{770}{1 - 0.20 - 0.10} = 1,100.00\ \text{m}^2$$

Rate conversion

Plinth area rate = Rs 3,400 per sq. ft; 1 m² = 10.7639 sq. ft, so rate = 3,400 x 10.7639 = Rs 36,597.26 per m². (The rate is used as printed in the question, in sq. ft.)

Estimate

Contingency 5% and supervision/work-charged 3% are assumed, since the question leaves them open.

Answer: Plinth area = 1,100.00 m²; total estimated cost = Rs 48,694,850.27 (about Rs 4.87 crore).

Plinth area

Carpet area = 800 x 1.2 = 960 m².

$$\text{Plinth area} = \frac{960}{1 - 0.20 - 0.15} = \frac{960}{0.65} = 1,476.92\ \text{m}^2$$

Estimate

Extras are percentages of the building cost; contingency and work-charged establishment are on the total cost (building plus extras).

Answer: Total estimated cost = Rs 7,057,292.31.

Plinth area

$$\text{Plinth area} = \frac{2000}{1 - 0.30 - 0.10} = 3,333.33\ \text{m}^2$$

Estimate

No contingency is given; 5% is assumed (usual practice). Supervision is 3% as given.

Answer: Plinth area = 3,333.33 m²; total estimated cost = Rs 76,320,000.00.

Plinth area and volume

Total carpet area = 4 x 250 = 1000 m².

$$\text{Total plinth area} = \frac{1000}{1 - 0.10 - 0.35} = 1,818.18\ \text{m}^2$$

Plinth area per storey = 1,818.18 / 4 = 454.55 m².

Height = 4 x 3.5 + 0.90 (parapet) = 14.9 m, so volume = 454.55 x 14.9 = 6,772.73 m³.

Estimate

The cube rate is used as printed (Rs 250/m³). The provisions for architecture 1%, water supply 6%, sanitary 6%, electrification 8%, contingency 5% and supervision 3% are assumed.

Answer: Volume = 6,772.73 m³; total estimated cost = Rs 2,212,650.00. If the rate is in fact Rs 2,500/m³, every amount is ten times larger.

Plinth area

$$\text{Plinth area} = \frac{3000}{1 - 0.20 - 0.10} = \frac{3000}{0.70} = 4,285.71\ \text{m}^2$$

(total over the six floors; 714.29 m² per floor.)

Estimate

Answer: Total estimated cost = Rs 133,928,571.43 (about Rs 13.39 crore). No contingency is given in the question; if required it is added on this total.

Plinth area

$$\text{Plinth area} = \frac{2500}{1 - 0.30 - 0.10} = 4,166.67\ \text{m}^2$$

(total of five floors; 833.33 m² per floor.)

Estimate

Answer: Total estimated cost = Rs 125,000,000.00 (Rs 12.5 crore).

Volume and building cost

Plinth area = 500 m², height = 16.25 m.

$$\text{Volume} = 500 \times 16.25 = 8,125.00\ \text{m}^3$$

Cost of building = 8,125.00 x 3,450 = Rs 28,031,250.00.

Additional items (percentages of building cost)

Answer: Total cost = 28,031,250.00 + 7,007,812.50 = Rs 35,039,062.50.

Assumptions

The question does not give the carpet area (and the wall percentage is struck out in the paper). I take an assumed total carpet area of 400 m² for the two floors (200 m² each), and the wall area as 10% of the built-up area, as in similar questions. The method is the same for any other carpet area $C$: plinth area $= C/0.60$.

Plinth area

$$\text{Plinth area} = \frac{400}{1 - 0.30 - 0.10} = 666.67\ \text{m}^2$$

Estimate

Answer: Plinth area = 666.67 m²; total estimated cost = Rs 14,774,400.00 for the assumed 400 m² carpet area. (Cost = 1.14 x 1.08 x 18,000 x C/0.60 = Rs 36,936 per m² of carpet area.)

Plinth area

$$\text{Plinth area} = \frac{500}{1 - 0.30 - 0.10} = 833.33\ \text{m}^2$$

(for the two floors together; 416.67 m² per floor.)

Estimate

Answer: Total estimated cost = Rs 11,250,000.00.