Hi Lisa! I'm building the Fairfield as well and had the same issues with tabs and slots not lining up properly. I had to extend some slots and do light sanding on a few tabs. It's recommended that you build the main dollhouse structure first without glue (dry-fitting). Use painters tape to hold the pieces together. This allows you to see how everything fits (or doesn't). It also helps with planning how and when you do the decorating of the tights spaces like the entry/tower areas and if you want to make any modifications to the design.
I've taken my house in and out of dry-fit multiple times. Only my foundation and chimney are actually glued so far. In the attached photo you can hopefully see what I did with the central wall piece. Notice how long the horizontal slots are next to the doors.