# Linear and Nonlinear Modelling of the Usable Area of Buildings with Multi-Pitched Roofs  
[PERSON]  
1 Faculty of Civil Engineering, Environmental and Geodetic Sciences, Technical University of Koszalin, 75-453 Koszalin, Poland; [EMAIL_ADDRESS]  
2 Faculty of Mining Surveying and Environmental Engineering, AGH University of Krakow, 30-059 Krakow, Poland; [EMAIL_ADDRESS]  
3 Institute of Economics and Finance, University of Szczocin, 71-101 Szczocin, Poland; [EMAIL_ADDRESS]  
4 Faculty of Agriculture and Forestry Ruralia Institute, University of Helsinki, 60320 Senajoki, Finland  
5  
[PERSON]  
2 Faculty of Mining Surveying and Environmental Engineering, AGH University of Krakow, 30-059 Krakow, Poland; [EMAIL_ADDRESS]  
[PERSON]  
3 Institute of Economics and Finance, University of Szczocin, 71-101 Szczocin, Poland; [EMAIL_ADDRESS]  
4 Faculty of Agriculture and Forestry Ruralia Institute, University of Helsinki, 60320 Senajoki, Finland  
5  
[PERSON]  
4 Faculty of Agriculture and Forestry Ruralia Institute, University of Helsinki, 60320 Senajoki, Finland  
5  
Footnote 10 studies revealed deficiencies in the REPR, especially concerning data on the usable areas of residential buildings [14; 15; 16]. For example, a study involving 800 residential buildings covered in the REPR in the counties of Koszalin and Kolobrzeg revealed that information on usable area was available only in about 40% of properties [16]. This limited data availability can significantly hinder the search for comparable transactions. Particularly in small local markets, with only few similar property transactions, real estate appraisal using the comparative method is therefore highly questionable. Calculating a building's usable area is also a complex issue. Several publications [17; 18; 19; 20; 21] show that the standards named in the Polish legislation that specify the rules for measuring and defining usable area are rather complicated. Until 1999, the usable area was commonly calculated according to the PN-B-02365:1970 standard [22]. Calculating the usable area according to this standard included only spaces that can be used for their intended purpose, including living rooms, kitchens, and bathrooms. However, when measuring the usable area, it is also important to consider the slopes of the roof in properties with an attic. In this case, different regulations apply depending on the height of the walls. Another common standard was PN-ISO 9836:1997 [23], which considered spaces such as basements, boiler rooms, garages, or other utility spaces that were not used directly for living not applicable to usable area. In the years 1999-2012, both standards were in force. After 2012, the PN-ISO 9836:1997 standard became mandatory for newly built properties. Over time, additional rules were also introduced which significantly affected the method of calculating the usable area [24]. Furthermore, the Polish Committee for Standardization withdrew PN-B-02365:1970 and PN-ISO 9836:1997, and since 2015, the recommended standard is PN-ISO 9836:2015-12 [25]. This standard introduced updated definitions and measurement procedures consistent with international standards. This standard specifies the method of calculating the usable area on different floors, including sloping ceilings and attic space. The standard places particular emphasis on the precision of measurements in residential and commercial buildings and eliminates potential discrepancies in the interpretation of previous guidelines. The regulation of usable areas introduced changes in the way indoor spaces are measured and classified. The updated rules also consider changing construction technologies and the way real estate is used. The standard has been adapted to European standards, which makes it easier to compare properties within European Union markets. An additional difficulty for real estate appraisers in Poland using the REPR is the lack of information regarding the standards the usable area of the building was calculated by.