This paper focuses on an analysis of the constitutional principle of legality in the context of the legal bases for issuing decisions on development conditions. Particular attention is devoted to their significance in legalisation and remedial proceedings conducted pursuant to Articles 48–49 and Articles 50–51 of the Building Law Act. The aim of this study is to assess the admissibility of issuing decisions on development conditions for existing developments and their impact on building control processes. It further discusses the investment process in light of the Build-ing Law Act, emphasising the requirement to ensure that developments comply with local spatial development plans or administrative decisions. The legalisation procedures are examined in detail, with attention to their formal and legal conditions, as well as the consequences for investors and administrative authorities. A key aspect of the discussion concerns the issue of decisions on development conditions as an instrument enabling the legalisation of building structures. The paper also addresses doubts related to their issuance, including the potential application of such decisions in legalisation proceedings and their effectiveness in restoring spatial order. The concluding section of the paper presents de lege ferenda proposals, highlighting the need for reforms in the legalisation system and spatial planning framework. The suggested reforms aim to enhance the transparency and efficiency of administrative procedures in the areas of building control and spatial order protection.