This article presents the characteristics of the simplified legalisation procedure concerning unlawful building work which has been in force since 19 September 2020 in Poland. The newly-introduced solution will enable to make the buildings or their parts made unlawfully more than 20 years ago legal in a significantly simplified scope, beneficial for the building owners and administrators. The so-called old unlawful building work can be legalised in this procedure, provided complete legalisation documents are submitted, with no need to pay the legalisation fee and even when the buildings or structures do not comply with the area development plans. However, legalisation is possible solely if it is verified that the technical condition of the building or structure does not pose any hazard to human health or life and ensures its safe use. The article discusses the assumptions of the amendment and summarises the introduced regulation.