First step is to look up the property on the Zoning map to see what you can build by right and that you have the appropriate property line setbacks.
If so, then it can be a good next step to familiarize yourself with the building inspectors and the process.
If not, then it's best to go to an experienced Zoning attorney and discuss whether it's possible and economically feasible to try to build a 3 family on the site. They can help you understand the potential costs, timelines, and hurdles. Neighbors carry a lot of weight if you need to go through Zoning appeals so knowing if the area is amenable to your project first will save you time and money.
Supposing that goes well, then you're on your way.
Looking at construction costs alone, a new build can be cheaper. But in Boston there are other factors that could hit your pocket harder before construction starts.
Good luck 👍
