PurposeThe concept of an open floor plan has become ubiquitous in modern residential design and is often associated with optimizing usable space and increasing spatial efficiency. However, one of the significant factors that has not been thoroughly explored in the context of the sustainability of open floor plans is the plan openness index, originally introduced in this research paper and defined as the ratio of open plan area to apartment gross area.Design/methodology/approachThe methodology focuses on analyzing the impact of the internal open-plan layout on the sustainability of residential spaces, utilizing the selection of 20 characteristic single-level residential spaces with varying interior design approaches, digitally drafted in AutoCAD software for area calculations and the plan openness index, followed by a comparative analysis.FindingsThe conclusion of the study emphasizes that internal openness of the layout is defined by newly introduced parameters (plan openness index, degree of function integration, open plan expandability, unbuilt wall area) is crucial for the sustainability of residential spaces, enabling a more efficient flow of light, air, and energy, adaptability of space, reducing the ecological footprint and extending the lifespan of the dwelling, with higher levels of openness typically resulting in greater sustainability.Originality/valueThis study introduces a novel approach to evaluating the sustainability of open floor plans by defining and applying new quantitative parameters. By systematically assessing their impact, the research provides a structured framework for optimizing residential layouts in terms of spatial efficiency, adaptability and resource conservation. These findings contribute to the broader discourse on sustainable housing design and offer a methodological foundation for future studies in this field.