Types of Estimate in Civil Engineering: Approximate, Detailed, Revised and Supplementary

Civil engineering uses about nine named types of estimate, and they fall into two families. Approximate estimates put a number on a building before its drawings are final, while detailed estimates price every item of work from those drawings, and the revised, supplementary and annual repair estimates are detailed estimates made later in a project’s life.

Knowing the types of estimate in civil engineering helps anyone who builds. A bank, a contractor and a government tender all use these words, and each of them means a particular document with a particular level of accuracy. A plinth area figure is good enough to decide how many floors you can afford. It is not good enough to sign a contract on.

There are no rupee figures in this guide. Rates change every year and differ from town to town. The methods do not, and the two documents that define them for government work in India, the CPWD Works Manual and the CPWD Plinth Area Rates, are quoted below.

How many types of estimate are there in civil engineering?

Books count them differently. Some list plinth area, cube rate and service unit as types of their own, others as methods for making one preliminary estimate, so one syllabus says five types and another says nine. The table puts all of them in one place, in the order a project meets them.

The CPWD Works Manual 2019 adds one more name, the enabling estimate. It pays for the work done before construction can start, such as concept planning and fixing the exact scope, and the manual says it must still be followed by a proper preliminary estimate.

What is an approximate estimate, and why is it made first?

An approximate estimate gives the cost of a work before anyone has measured it item by item. The CPWD Works Manual 2019 defines the preliminary estimate as one “based on plinth area cost basis with CPWD Plinth Area Rates (PAR) or any other method without detailed items of work”, made to obtain administrative approval and expenditure sanction. In plain words, it is the number a department uses to decide whether the work should happen at all, and how much money to set aside.

The rough cost estimate comes even earlier. CPWD prepares it only on the client’s request, from whatever the client has described, to give “a ready approximate cost”. Neither of these needs a full set of drawings.

A preliminary estimate is usually shown under separate heads for land, each building, roads, water supply, sanitary work and electrification. A short report explains why the project is needed, and a site plan goes with it. For a project that earns money, such as a rented building, the same figure is set against the expected income to see whether the investment makes sense.

For a family building a house, the approximate estimate does the same job on a smaller scale. It tells you whether to build one floor now and one later, and it is the figure you take into a first conversation with a bank.

What are the types of approximate estimate?

Three methods are taught in Indian estimating courses. Each takes a rate from a similar building finished recently in the same locality, to the same specification, and multiplies it by a measure of the new building.

Plinth area method. The plinth area of each floor is multiplied by a plinth area rate. CPWD publishes its own rates per square metre, split by building type (office, school, hostel, residential) and by floor height, with separate additions for things that raise the cost. In PAR 2012 these additions included every storey above six, every 0.3 m (about 1 ft) of extra floor height, foundations deeper than the normal 1.2 m (about 4 ft), weak soil, pile or raft foundations and earthquake resistance. Water supply, sanitary and electrical work are then added as a percentage of the building cost.

Cube rate method. The building’s volume is multiplied by a cube rate. Length and breadth are measured outside the walls at floor level. Height runs from floor level to the top of the roof, or halfway up a sloping roof, and for storeyed buildings from one floor level to the next. Foundations, the plinth and the parapet above the roof are left out. Because height is counted, this method is more accurate than the plinth area method for buildings with tall or unequal floors.

Service unit method. Some works are easier to price by what they serve. A school or hostel is costed per student, a hospital per bed, an auditorium per seat, a water tank per litre stored, a road per kilometre and a culvert or bridge per metre of span. A hostel for 180 students, at the cost per student of a similar hostel built recently nearby, gives its figure in one multiplication.

How is plinth area measured for an estimate?

Plinth area is the built-up covered area measured at the floor level of each storey and the basement, as defined in clause 2.1 of IS 3861:2002, the Indian Standard for plinth, carpet and rentable areas. The same standard does not accept terms like “super built-up area”, which is worth remembering when a builder’s brochure uses one.

For estimates, CPWD prints its own measuring rules alongside its plinth area rates, from a 1983 memo, so that every engineer works out the area the same way:

• The total plinth area is the sum of every floor, basement included.
• The ground floor is measured at plinth level, ignoring plinth offsets up to 2¼ inches (about 57 mm). Where columns stand outside the cladding, measure to the face of the cladding.
• A verandah with a parapet that is protected by a roof counts in full. An unprotected one counts at 50%.
• Unprotected balconies count at 50%. Cornices and sunshades do not count.
• The mumty (the staircase cabin on the terrace) does not count. Open platforms, terraces and porches do not count either, but their cost is added separately.
• In a house, internal sanitary shafts are left out. In an office or hospital, shafts and lift wells are included.
• A mezzanine floor counts. A storage loft with no staircase does not.

The house construction cost calculator explains how plinth area relates to the carpet area and built-up area that sellers and banks quote.

Worked example: plinth area and cube rate for a two-storey house

Take a house with a ground floor and a first floor, each 10 m × 12 m outside the walls. The first floor has an open balcony 1.2 m × 4 m with no roof over it, and the terrace has a 3 m × 3 m mumty. Each storey is 3.2 m (about 10.5 ft) from floor to floor. Call the local plinth area rate R per m² and the local cube rate C per m³. Both come from a recent house of the same specification nearby.

Put your own local rates in place of R and C and the estimate is done. The one rule that matters: measure your building the way the reference building was measured when its rate was worked out. A rate built on a plinth area that counted balconies in full will overstate the cost of a house measured by the CPWD rules, and the other way round.

A rate from an older schedule also needs bringing forward. CPWD issues its plinth area rates against a base cost index of 100 on the date of issue, and later estimates multiply the rate by the current cost index ÷ 100. That index is a weighted basket of materials and labour. In PAR 2012, labour carried 25% of the weight, steel 19.5%, cement 14.5%, bricks 8% and 20 mm aggregate 6.5%, with sand, flooring, paint, wood, pipes, fittings and wiring making up the rest.

What is a detailed estimate, and what goes into it?

A detailed estimate prices every item of work separately. CPWD defines it as a “complete and comprehensive estimate” based on the Schedule of Rates for the items the schedule covers and on market rates for the rest, “supported by detailed drawings and specifications”. It is made after the project is approved, once the site is free to build on. Its approval is called technical sanction, which certifies that the estimate rests on a properly detailed design.

It is built in two stages:

1. Details of measurement. Each item, such as excavation, concrete, brickwork, plaster or paint, is measured from the drawings, with length, breadth and height written out and the quantity worked out.
2. Abstract of cost. Each quantity is multiplied by its rate, and the amounts are added under each sub-work (main building, boundary wall, septic tank and so on).

A small percentage is added for contingencies, usually 3% in textbook practice, and a provision for work-charged establishment, meaning the site supervisors and watchmen paid from the work itself. A report, the general and detailed specifications, the drawings, the design calculations and a rate analysis for any item not in the schedule go with it.

Leave the rate and amount columns of the abstract blank and you have a bill of quantities, the document contractors price in a tender. The units, measuring rules and rate analysis behind it are covered in the construction estimate format guide, and the material side in the building material calculation guide.

Machines enter a detailed estimate through the rates of the items they do. Foundation excavation dug by a JCB 3DX backhoe or a Tata Hitachi EX 210 Infra crawler excavator is priced per cubic metre, with machine hire and fuel inside that rate. This year’s figures for that item are in the excavation rate per cubic metre article. If you are planning the earthwork, compare backhoe loader models and connect with a dealer.

Approximate estimate vs detailed estimate: what is the difference?

The approximate estimate asks “roughly how much, and should we go ahead?” The detailed estimate asks “exactly what, how much of each, and at what rate?”

The tender row comes from the Works Manual. An EPC contract cannot be priced item by item because the contractor does the design, so CPWD works out its estimated cost from plinth area rates adjusted by the cost index.

When is a revised or supplementary estimate needed?

A revised estimate is a fresh detailed estimate for work already sanctioned whose cost has moved. Estimating textbooks and state works-accounts rules give three triggers:

• the sanctioned estimate is, or is likely to be, exceeded by more than 5%, from higher rates or any other cause;
• spending is likely to exceed the administrative sanction by more than 10%;
• the work has deviated materially from the original proposal, even when the money still fits.

For CPWD works the 2019 Works Manual sets the bar at the administrative approval amount. On deposit works a revised estimate is submitted once spending runs more than 10% above it, and nothing above it may be spent without the client’s approval. A revised estimate carries a comparative statement listing each item’s original and revised quantity, rate and cost side by side, with the reason for each change.

A supplementary estimate is different. It is a detailed estimate for new work that was not in the original estimate but became necessary while the project was in progress. Its abstract shows the original amount, the supplement, and the total for which sanction is now sought. When part of a work is dropped and what remains costs under 95% of the original sanction, a combined supplementary and revised estimate is prepared and a fresh technical sanction obtained.

The same ideas apply on a private site. When a contractor says “extra item”, the Works Manual’s own term for an item that is new and not in the schedule of quantities, ask for a separate priced note, as a supplementary estimate would give. When rates rise across the board, ask for the comparative statement.

What are annual repair and special repair estimates?

Chapter 9 of the CPWD Works Manual splits work on existing buildings into seven kinds: annual repair and maintenance, special repairs, additions and alterations, maintenance of VIP residences, day-to-day repairs, petty repairs, and upgradation. Each gets its own estimate because each is approved and booked differently.

The annual repair estimate is a detailed estimate made each year to keep a structure in proper and safe order. For a building it covers whitewash, colour wash, painting and minor repairs. For a road it covers patch repairs, renewals and the repair of culverts and bridges. Some departments also make a monsoon damage repair estimate.

A special repair estimate covers major repair or replacement of part of a structure after a breakdown or serious decay, where a major component has to be replaced. The quantities for a repaint come from the same rules as new work, explained in the paint quantity calculation guide.

What does contingency mean in an estimate, and how much is added?

Contingencies are money kept in an estimate for small unforeseen costs that belong to the work as a whole but fit under no single item. That is the CPWD definition, put simply. The Works Manual lets this sum pay for a site office, watch and ward staff, surveying, material testing, estimating, structural design, drawings, models and an inspection vehicle, and, with the client’s approval, for small works that crop up without a provision of their own.

In a detailed estimate the usual allowance is about 3% of the estimated cost. It is not a cushion for rising rates. A rise of more than 5% calls for a revised estimate, not a dip into contingencies.

Which estimate do you need for your own house?

Start with a plinth area estimate. Measure the plinth area by the rules above, take a rate from a house of the same quality built nearby in the last year or two, and add services and a contingency. That tells you what you can afford and whether to build in stages.

Before you choose a contractor, get a detailed estimate. It does not need to be long for a house, but every item should have a quantity and a rate, so that two quotes can be compared line by line. Steel quantities come from a bar bending schedule, and small items such as the plinth beam are measured the same way.

Once work starts, keep the habit of the revised and supplementary estimates. Write down every extra item before it is built, and compare the running cost against the sanctioned figure. Most cost overruns on small houses are a string of extra items nobody priced.

Government works follow the same chain all the way to the tender notice. For item rate and percentage rate tenders, the estimated cost put to tender in the live notices on DesiMachines Opportunities comes from the detailed estimate. For the machines that dig and fill your foundations, compare excavator models and connect with a dealer. More estimating guides sit under construction cost.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.

Sources: CPWD Works Manual 2019, definitions (contingencies, detailed estimate, enabling estimate, extra item, preliminary estimate, rough cost estimate, special repair, technical sanction) and clauses 3.1.1.1 to 3.1.1.6, 3.1.4, 4.8(3) on EPC estimated cost, and 9.1; CPWD Plinth Area Rates 2012 (1.10.2012), Annexure I (extras and services), Annexure III (Memo 29/21/58/WI of October 1983, rules for working out plinth area) and Annexure IV (cost index weightages); IS 3861 : 2002 (method of measurement of plinth, carpet and rentable areas of buildings), clauses 2.1 and 4. Cube rate, service unit, detailed estimate, revised and supplementary estimate practice as set out in Indian estimating and costing course material (Sathyabama Institute, Institute of Aeronautical Engineering) and the Punjab State Transmission Corporation works accounts examination paper of 2024. The worked example uses symbols in place of rates; your local rate and your own drawing decide the final figure.