The exclamation mark on the buildings icon is a bug which I think is associated with appalling Expedition side event.
It does (at the moment) not indicate a new building for you to buy. If you look carefully at the harbour view, I think you will see a lock icon. If you click on it, it will open a dialog box offering you more land ( at a cost) to expand your port.
There are two views on this.
- It is unnecessary, as with some creative moving of buildings you can squeeze all the buildings onto the land you already have. Note : You cannot spin buildings around.
- It allows a more spacious layout. Personally I prefer option 2
In either case buying the extra land does not mean you get more buildings.
The maximum number of buildings you get are as follows
Main dock x 1
Fishing dock x 1
Town Hall x 1
Museum x 1
Workshop x 1
Tavern x 1
Lighthouse x 1
Houses x 6
Warehouses x 8
Lumber Mill x 1, Sawmills x 2
Forge x 1 . Foundry x 2
Mine x 1 . Quarry x 1
Grocery, Yard, Food Mill each x 1
Total number of building 32
Hope this helps
