Under Workplace Safety and Health (First-Aid) Regulations section 6, a first aid room is legally required when the company has more than 500 persons at work, which includes contractors. 

For some companies, they do not have less than 500 people working during normal periods.

But instead, they may experience sudden influx of contractors coming into the company during peak periods for maintenance which boost the workers at work to more than 500.

So for such cases, they can consider writing to the Ministry of Manpower to ask for permission case by case to obtain a waiver of this requirement rather than fork out a space for the first aid room. 

Quoted from Workplace Safety and Health (First-Aid) Regulations