I googled on "fcc fm transmitter" and looked at several websites and publications that seemed to be credible sources of information. I read that for unlicensed operation within the FM broadcast band that the limit is based on 250 microvolts per meter of field strength at 3 meters from the transmitter (other conditions apply). One of the sites showed an example with calculations that suggested 1 watt was several orders of magnitude more power than might be allowed. Each country has it's own rules but the US rules provide some insight.
Based on what I read and a little bit of experience I'd suggest that you could purchase a certified compliant (for your location) commercially available transmitter that includes an antenna and use it as indicated by the manufacturer. You are responsible for the emissions but it's likely that you'll be ok. If you choose to build your own then you accept the responsibility for design and operation.
In the US, and possibly other countries, they do not have large numbers of radio police to monitor this kind of activity. There are however, lots of people who could be affected by a transmitter and if you bother them enough they will find you and if required, ask the authorities to enforce the regulations. This might suggest that if you live in a rural area and use very low power, that the odds of bothering anyone are relatively low - and then as the population density increases, the odds of bothering someone increases. One could also argue that there is so much junk in the RF spectrum it might be tough to single out any one offender - though I can say that I've worked with teams locally to track down spurious emissions - usually but not always some malfunction.