(96)Those sector-specific measures should be complemented by an adaptation of Directives 2004/17/EC and 2004/18/EC empowering contracting authorities to pursue the objectives of the Europe 2020 strategy for smart, sustainable and inclusive growth in their purchasing strategies. It should hence be made clear that, except where it is assessed on the basis of price only, contracting authorities can determine the most economically advantageous tender and the lowest cost using a life-cycle costing approach. The notion of life-cycle costing includes all costs over the life cycle of works, supplies or services. This means internal costs, such as research to be carried out, development, production, transport, use, maintenance and end-of-life disposal costs but can also include costs imputed to environmental externalities, such as pollution caused by extraction of the raw materials used in the product or caused by the product itself or its manufacturing, provided they can be monetised and monitored. The methods which contracting authorities use for assessing costs imputed to environmental externalities should be established in advance in an objective and non-discriminatory manner and be accessible to all interested parties. Such methods can be established at national, regional or local level, but they should, to avoid distortions of competition through tailor-made methodologies, remain general in the sense that they should not be set up specifically for a particular public procurement procedure. Common methodologies should be developed at Union level for the calculation of life-cycle costs for specific categories of supplies or services. Where such common methodologies are developed, their use should be made compulsory. Furthermore, the feasibility of establishing a common methodology on social life cycle costing should be examined, taking into account existing methodologies such as the Guidelines for Social Life Cycle Assessment of Products adopted within the framework of the United Nations Environment Programme.