If you have a walkway and then use it as an emergency escape route you need to go by the set sizes as laid down in the building regs Current building regulations contain guidance on the widths of escape routes and exits for new-build, non-domestic properties and the communal areas in purpose built blocks of flats in “The Building Regulations 2010, Fire Safety, Approved Document B, Volume 2 – Buildings Other Than Dwellinghouses, 2006 edition, incorporating 2007 and 2010 amendments”. The following information is extracted from page 37 of the above document: 3.18: The width of escape routes and exits depends on the number of persons needing to use them. They should not be less than the dimensions given in the following table: 3.20: Widths of escape routes and exits Maximum Number of Persons Minimum Width (mm) 60 750 110 850 220 1050 More than 220 +5mm per person In calculating exit capacity, the document notes the further points: 3.21: If a storey or room has two or more storey exits it has to be assumed that a fire might prevent the occupants from using one of them. The remaining exit(s) need to be wide enough to allow all the occupants to leave quickly. Therefore, when deciding on the total width of exits needed according to the above table, the largest exit should be discounted. 3.22: The total number of persons which two or more available exits (after discounting) can accommodate is found by adding the maximum number of persons that can be accommodated by each exit width. For example, 3 exits each 850mm wide will accommodate 3 x 110 = 330 persons (not the 510 persons accommodated by a single exit 2550mm wide). Hope that helps

500mm seems rather narrow to be doing anything other than shuffle through (and that's for a slim person). If the inspection needs anything more than a cursory glance at eye height then I suggest that the suggested width is inadequate, quite regardless of any legal issues.