I'm planning for a small addition that will include one full bathroom. The existing house (1950's ranch) has 3/4" copper supply lines.

Does anyone know whether Massachusetts code will allow me to tie the addition bathroom into the existing 3/4" supply system? Or does an addition require an increase in supply pipe diameter all the way back to the water main connection?

The reason I'm asking is that I need to repair some of the supply lines in the area of the planned addition, and I'm wondering whether I should ask my plumber to leave stub connections for the addition bathroom, or whether it will all need to be replaced to meet code when the new construction happens.

Thanks for your help!

YOur building department is the one to answer that question, not someone on the Internet.

being a 1950's ranch, you probably have a 3/4 copper water service coming into the house.
so, supplying the new bath off a 3/4 copper main is about the best you are gonna get.

Yes ask your plumbing to plumber 3/4 lines to future bathroom and if you plan on having a house full of people and a huge demand in water during peak times then upsize your water main to at least 1" and better 1 1/4"

If you're not a licensed plumber, Massachusetts code won't allow you to do anything,

Follow the local code for sure but water pipe size is determined by distance, fixture Unit values and existing pressure. When you inquire at the city have all of this information available to help your inspector answer your question. good luck.