Access to and use of all activities in the built environment to ensure equal participation for people with disabilities is a basic human right, recognised globally in the Convention on the Rights of Persons with Disabilities, as well as a fundamental objective in an increasingly ageing society.The Spanish Act on equal opportunities, non-discrimination and universal accessibility (Ley 51/2003) recognised such challenge and established the obligation to ensure accessibility in the built environment 'liable to reasonable adjustment'. The deadline for meeting that obligation was 4 December 2017.This paper formulates a methodology to assist policy makers and urban planning authorities to define the legal concept of 'reasonable adjustment' in relation to small establishments within the existing built environment. No criteria or tools have yet been forthcoming in this area despite the deadline for compliance with the provisions of the act. That regulatory gap exacerbates the complexity in applying and interpreting the legislation on adaptations of the built environment.Starting from a legislative and technical synopsis of the international implementation of the term 'reasonable adjustment' and based on the analysis of 120 works in establishments of Madrid and its metropolitan area, the proposed methodology addresses the characterisation of: flexibility requirements; accessibility adjustments; economic viability by activity involved; and factors affecting overall accessibility.Regardless of the designers and owners ability, the research shows that regulatory developments, urban planning and its control by Public Authorities, both local and national, have facilitated the accessibility improvements. In terms of accessibility requirements, even today the difficulty of accessibility compliance is focused on wheelchair users. In the analysed case study, the investment in accessibility means about 0.5 % of the property values.