There is no blanket minimum size for a building certification. For investors and project developers, certification already pays off for medium-sized projects, since certified properties often have better marketing prospects and, under certain market conditions, can achieve higher rental and sale prices. For public buildings or subsidy-financed projects, the question of size is secondary anyway, as a certificate is often a funding requirement or part of sustainability requirements. What matters above all is the right timing: the earlier certification is integrated into the planning, the lower the additional effort.

Yes, this is fundamentally possible. Systems such as BREEAM are explicitly designed for existing properties and enable step-by-step improvement. DGNB and LEED also offer corresponding schemes for building operation. The decisive factor is that both the technical systems and the operating processes demonstrably meet the requirements. Without valid consumption data from real operation, retrospective certification becomes significantly more demanding.

The cost of a building certification varies depending on building size, use, and the desired certification level. In addition to the registration fees of the certification body, you should also factor in the auditor's fee as well as the internal effort for documentation and verification. In practice, the pure certification costs are often in the low single-digit percentage range of the construction costs, or in the five- to six-figure euro range.