Historical masonry structures are exposed to environmental stressors and ageing-related degradation, which lead to spatial variability in mechanical properties. Their conservation requires accurate structural assessment strategies integrating numerical models with site-specific knowledge. However, current engineering practice often neglects deterioration mechanisms such as moisture ingress, salt crystallisation, and poor construction quality. This paper presents a structured questionnaire designed to develop minimally invasive in-situ investigations, ensuring sufficient knowledge of the spatial variability of key mechanical properties. Aimed at conservation professionals, the tool guides participants through the design of a site campaign for a Simulated Physical Asset—a historical limestone masonry façade affected by typical deterioration patterns—using graphical inspection tools, predefined NDTs/MDTs, and cost constraints. Participants define the location and number of tests to estimate mechanical properties and justify their decisions, enabling the assessment of how expert judgment influences model reliability. Future developments will prioritise geostatistical mapping of deterioration to inform simplified micro-modelling strategies for historic masonry structures. This integrated approach captures the spatial variability of material properties and underpins predictive modelling workflows aligned with ICOMOS principles for the sustainable conservation of built heritage.