## 2 Criteria Affecting Stair Safety and Sustainability in a Single-Family House  
Stair safety and sustainability in a single-family house might be affected by various criteria, including environmental factors, design guidelines, and maintenance. A well-designed, safe, and aesthetically pleasing staircase that blends harmoniously into the interior of the house and contributes to the overall comfort and functionality of the building, represents the general requirements frequently expressed by the house owners or residents. Homeowners' involvement in the staircase-related decision making involves many different aspects: expressing preferences for staircase style, materials and finishes, setting a project budget, considering specific safety and accessibility features, and assessing how the staircase project fits into their daily lives. Consideration of environmentally friendly materials, natural lighting, effective space planning and the long-term maintenance may also be priorities of the homeowner. However, such important aspects as effective space planning, structural integrity, natural lighting, stair accessibility, aesthetics, and design, as well as the decision compliance with building safety regulations, are the responsibility of architects. Therefore, it is very important to ensure effective cooperation between architects and homeowners when making decisions about the installation or renovation of internal stairs. Raising awareness of inside-stair safety and criteria affecting sustainability may also help in achieving the best possible decision on the selection of staircase projects. Based on state-of-the-art analyses and expert consultations, seven criteria have been identified as key factors contributing to safety and sustainability of inside stairs installed in a single-family house. These are staircase material, handrail construction, stair width, riser height and tread width, step coating, and natural lighting. Each of them can be described both from the safety and sustainability perspective. Staircase materialThe choice of material for the construction of a staircase can be linked to safety, aesthetics and environmental friendliness. Staircases installed in detached houses are usually made of wood, steel, or reinforced concrete. All these types of staircase can be robust and aesthetically pleasing, when designed and constructed using modern technologies. Wooden stairs are widely used in single-family houses to create a natural and warm atmosphere. They are appreciated for their aesthetics and comfort, harmony with the overall style of the house, safety, and compliance with space requirements. Since the production (and subsequent recycling) of a wooden staircase might be more sustainable compared to metal and reinforced concrete [41; 42], its usage can be seen as an opportunity to reduce negative environmental impacts [43]. Steel staircases can be chosen to achieve a more modern, light, and minimalist staircase design. Reinforced concrete stairs are typically chosen where strength and durability are a priority, or where a distinctive design is required. However, this alternative is more expensive than others and may require more time and skill to construct. Handrail constructionHandrails act as a physical barrier, helping prevent accidental slips, trips, or falls on stairs [14; 15; 44]. They promote safe movement and confidence in stair navigation. When talking about handrails, it is important to ensure that they should be chosen considering the height and width of the staircase, as well as the specific needs of the house residents. The fastening and infill of the handrails also affect the safety of the stairs. The top handrails are easier to grip; they also provide a continuous surface throughout the entire length of the staircase. The side handrails can be a good option where space is limited or the wall is not strong enough to support a top handrail. When analysing handrail fastening, it is important to consider the type of infill chosen.