By state and federal law, the MBTA is extremely limited as to what improvements can be made to non-accessible stations without triggering the requirement for accessibility modifications to be made during those improvements. If the work is less than 30% of the replacement cost of the station, the improvements themselves must meet accessibility codes; if it's over $100k, an accessible entrance must also be provided. If it's over 30% of the replacement cost, the whole facility must be renovated for accessibility. (See
MAAB requirements; the 1990
MBTA Guide to Access has some old numbers but is very useful.) There is a specific exception allowing up to $500k in "general maintenance and on-going upkeep" for underground transit stations, provided the work does not involve the entrance.
What it takes to trigger accessibility modifications can be surprisingly small. The MBTA has been dilly-dallying for a while on dealing with the B and C surface stops - but
whoops: " Throughout 2018-2019, the MBTA conducted track replacement along parts of the Green Line’s B and C branches. As part of that work, portions of the platforms were removed and replaced – motivating the need to make them accessible." Now, the MBTA has to make a dozen B and C stops accessible by about 2024. (In this case, it appears that poor planning meant that the accessibility requirement was not noticed until afterwards. For a normal station renovation, it would be determined in the course of planning and getting permits.)