As long as you keep the two separate, it shouldn’t impact the home’s value. If anything, it can increase the overall cost and value of the heritage home. After all, you are occupying more land and creating a place worth living.
In most cases, adding an ensuite, like bathrooms, to small storage spaces can further augment the place’s value. Paying attention to plumbing, wiring and such would also help maintain and upkeep the value.
Apart from extensions, if you can invest in the maintenance, repairs, and improvements for the heritage home, it will further add to the overall value.