Today, many design proposals exists that address different strengthening issues. Most proposals are derived on assumptions made for designing of new structures and are also based on a deterministic approach where a safety factor is added. The use of probabilistic methods is now extending and the reliability is sometimes calculated afterwards for the strengthened structure. This paper presents how the reliability should be chosen on beforehand when doing design for strengthening of an existing structure, which issues to be considered and also what safety that one can expect from a strengthened structure. Partial coefficients on material properties and loads are used to give a uniform treatment of risk of failure when strengthening of structures. When partial coefficients are chosen the reason for strengthening and the strengthening method may be considered to achieve an optimal strengthening with respect to structural safety and economy. The theoretical basis, the same as for Eurocode, has been used successfully in Swedish design codes for a long time. The results from the analysis indicate that strengthened with CFRP plate bonding has a very desirable effect on structural safety and reduces the risk of failure by considerable proportions.