We are prepared by a “taking off” process in which the cost of a building or other structure is estimated from measurements in the architect's drawings. These are used to create a cost estimate such as in regard to the square area in meters of walls and roofs, the numbers of doors and windows, and systems as heating, plumbing and electrics. Similar types of work are then brought together under one item, a process known as "abstracting".
Estimating books provide the relevant costs of the materials and labour costs of the operations or trades used in construction. As the rates for materials and labour change due to inflation, these books are frequently republished.
The practice historically of estimating building costs in this way arose from non-contractual measurements, taken off drawings to assist tenderers in quoting lump sum prices.
There are different styles of bills of quantities, mainly the Elemental BOQ and Trade Bills.