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One of the most misunderstood yet critical parts of a specification is stipulating the design life of the building. As soon as the subject arises, lawyers go into overdrive, clients wince and the cost consultant reaches for the correction fluid. The result is that many specifications do not adequately cover these requirements, resulting in contradictory and unco-ordinated documentation.

The confusion surrounding the correct method of specifying design life arises from two specific areas: first from a misunderstanding of the place of warranties and guarantees in construction contracts and second from a lack of appreciation of the relevant British Standard (7543). So what exactly are the requirements of each and where does the confusion arise? Generally, a guarantee is a promise to be responsible for the performance of a duty and the rectification of a failure, whereas a warranty confirms that the work to be carried out is suitable and appropriate. These responsibilities should be included in the contract between buyer and seller and should not, in my opinion, be incorporated in the specification, as this is intended to confirm what the buyer wants and/or what the supplier intends to provide.

The specification does, however, have to deal with design-life issues – those of standards and quality – and it is here that things seem to go awry. In normal circumstances, the contract is in place before the specification is written, although it does often include specification-related issues, resulting in statements such as "the design life of the building will be …" And it is here that the trouble and confusion starts, because often a lawyer or client does not relate the design-life statement to British Standard 7543. In fact, most seem to have never even heard of it, let alone understood it. This results in a situation in which general statements regarding BS compliance are made, and then contradicted.

BS 7543 deals with the durability of buildings generally, as well as their component parts. It clearly states design-life categories for different types of building, which often do not relate in any way to the client's expectation of use. And to make matters worse, clients and their representatives tend to believe that if the periods stated in the BS were used, a significant increase in construction cost would result, making unreasonable demands on the poor old contractor. Yet at the same time, clients are happy to make general references related to compliance with these standards. This is often because they confuse design-life requirements, which relate to the specific type of building required, and the warranty or guarantee requirements, which provide among other things a period of cost-free repair and replacement. This is a mistake.

As soon as design life arises, lawyers go into overdrive, clients wince and the cost consultant reaches for the correction fluid

BS 7543 categorises buildings into five types, depending on how long they are designed to last:

• Fewer than 10 years: temporary buildings
  
• 10-30 years: warehouses, internal office refurbishments and so on
  
• 30-60 years: industrial buildings and housing
  
• 60-120 years: new health and education buildings and high-quality refurbishments
  
• More than 120 years: civic and high-quality buildings.

Clients tend to restrict design-life requirements to below 60 years. Why? It may be a simple misunderstanding of what is meant by the terms design life and service life. The BS states that design life is the "period of use intended by the designer", which is confirmed to the client in the specification. The service life, however, is the "actual period of time during which no excessive expenditure is required on operation, maintenance or repair of a component or construction". It is the service life of components that relates to warranty and guarantee requirements, and must reflect the client's expectations and life-cycle costing analysis.

This apparent confusion between design life and service life is at the core of the problem. Design life is fixed to the building type but service life is flexible to suit the realities of the project.