The conservation of historical buildings via charters, general principles, methods for maintenance and repair of the buildings have been analysed in the international environment. Theories, approaches and conservation methods have been adopted along with the developing technology in relevance with the current conditions. Conservation is a process including consistent, coordinated and programmed activity of study, prevention (limiting situation of risk), maintenance (controlling the conditions of property) and restoration (direct intervention). In this regard, the significance of non-destructive tests on existing historical buildings –in the context of planned conservation- should be highlighted, and become widespread. In Turkey, which is extensively rich in historical building stocks, it is a difficult task to appraise the physical conditions of all historical buildings by non-destructive tests with a limited number of experts. On the contrary, appraisal of present conditions of building stocks and detailed diagnostics phase of the buildings are important in terms of conscious and sustainable conservation. The buildings under the effect of interior and ambient environmental conditions and decays developed on building envelope or structural elements causing from these conditions affect the structure and thermal comfort conditions in a negative way. Continuous monitoring of these actions causing decay and degradations prevent the structure to be under high risk and the necessity of performing heavy interventions on buildings. By the non-destructive test applications during monitoring, the buildings are prevented to be under high risks and the necessity for heavy interventions can be eradicated. In this research, Pertev Pasa Mosque -constructed by Architect Sinan-; Gebze Coban Mustafa Pasa Mosque -constructed in 1523 and known as the work of Architect Sinan but various names asserted as the architect of building according to resources-, and Fevziye Mosque -initially constructed by Architect Sinan in 1576 but then destroyed and re-constructed due to disasters in the past and finally heavily damaged on 17 August 1999 then re-constructed- are analysed. Three mosques are listed by national protection board and they were investigated without any damage to the buildings. All structures whose original construction dates back to 16th century were selected according to three criteria: Pertev Pasa Mosque and Fevziye Mosque are exposed to same climatic conditions due to their close locations to each other; on the other hand Fevziye Mosque display different material and construction system due to the subject of being reconstructed several times. Coban Mustafa Pasa Mosque, conserved well like Pertev Pasa Mosque in terms of material, construction system and architectural properties, is located in a different place with respect to others. The outcome and methods of the project 114K284, Non-Destructive Test Applications, in the Context of Planned Conservation, through Historical Mosques of Kocaeli: Coban Mustafa Pasa Mosque, Fevziye Mosque and Pertev Pasa Mosque”funded by TUBITAK, are presented in this paper. Temperature and relative humidity values of inside and ambient environments are were measured by dataloggers and probes for three mosques. The visual and quantitative data of the buildings were obtained to appraise degradation of building envelope and define micro-climatic conditions of the materials by Infrared Thermography and Ultrasonic Velocity Methods. Condensation risk in the mosques by logging inside and ambient environment temperature and relative humidity level were found high considering praying times. Existing and possible deteriorations and previous interventions were found via infrared thermography captures. Moreover, the voids inside the building envelope and qualitative properties of materials were revealed by tests.