@Omegaexdeath
I don't know if it made it to ServUO, but someone released a Cathedral addon on RunUO awhile back. Everyone that tried to place the addon crashed the moment it was dropped. The item count was crazy, well over 20,000 pieces.
Here is what I've learned when it comes to building large structures:
If you're building a structure with multiple levels for your shard - Building each level as it's own addon will help keep the item count under 20K. Then place it on a map for your shard, place a level, then freeze it, place the next level and freeze it, rinse, repeat as needed.
If you're building a structure with multiple levels for public release - Same as above, but just keep each level as it's own addon file, zip it and release it.
If it's just a huge building - Split it up and down. Think of a cheese slicer...you're building the addon in sections, but not levels.
In any case, please remember that while it helps during the building process to perhaps start with a solid foundation, you don't NEED the whole foundation once it's completed. Since there's no such thing as First-Person View in UO, certain blocked areas don't even need to exist (they are just extra pieces contributing to a higher item count).