Estimating methods for interior jobs largely derive from the Architectural profession, which in turn have originated from Civil Engineering. However, gradually distinct methods for estimating interior jobs are evolving. Civil structures are large entities but made from few basic raw materials, compared with Architectural projects which consist of more raw materials and are complex. Civil and Architectural components as produced on the site are comparatively simplistic and require services of one or few contractors. Architectural components are generalized, traditional, and are being used for ages. Complex architectural systems are procured in full or partially ready state at workshops.
However, Interior jobs are composed of many but rarely recurring components. These are produced from many different materials. Complex interior systems composed or assembled on the site, are very intricate and require services of several contractors, craftspeople, etc. Interior components are custom made or improvised on a situation to situation basis. In interior jobs due to fewer occurrences, or total absence of standardized components, modules of measurements and modes of measurements are not very useful. Rate analysis and other systems for determining an appropriate market rate per item/ sub item / task are not suitable.
Civil structures and Architectural buildings are executed by third party contractors and, so require detailed estimates. However, in many design fields including Interiors, Design+Build or design and execute type of job handling is very common. Design+Build interior jobs are paid per the total cost (design services + cost of execution + profit), so a detailed estimate or item wise cost estimate is not required, except for the in-house judgements.
Interior elements are of many different varieties: paints and polishes, soft furnishings, furniture, machines, electronic systems, hard furnishings, building elements. For accounting purposes (by clients) some of these elements are classified as recurring expenses, while others qualify as asset creating investments. Asset creating investments are written off (discounted) for short or long term depreciations. Clients wish to take maximum benefit through tax planning, and so demand bills to suit the accounting needs. Estimate schedules reflect the grouping of items to this end.