If the building is 9cm wide, an island platform would probably need to be more than 15cm wide. There has to be a minimum of 6ft between the platform edge and any fixed structure (buildings, railings, lampposts, etc). 6ft x 4mm = 24mm x 2 = 48mm. Of course, 9cm + 4.8mm is only 13.8mm but that minimum measurement rarely applies. You would normally expect at least 10ft which would give a platform 17cm wide.