All grandfathered homes may be customized as long as you don't change the foundations.
The exception being L-shaped and towers. L-shaped can't be customized without changing
the footprint (foundation) which nulls grandfather status. Towers have the overhangs so
customization isn't available either.
All the old 7x7 may be customized as I have 2 on Atl (alt homes for chars). I made them
3 story then added a 4th goza mat floor for garden sheds, lighthouses, map tables, etc,...
It should be noted that when customizing the 7x7s you will lose one space in front as the
game sees your steps (whether you have them or not) as potentially 'your' house. This will
effectively make your home a 6x7 on the 1st floor (6 being north to south). The original
7x7s by today's game standards were actually 8x7 as the game did not view steps as being
part of the house (compared to customized homes).
From what I can remember, original there was no limit to homes owned (as I lived out of
2 blue and 2 green tents all placed by the same char back in the day). Then it went to one
home per character per shard. Shortly after, it changed to one home per shard then one home
per account. Of course now, as stated, its 2 homes per account as long as one is on Siege.
I remember for months having to manually refresh all my homes across all shards because
they didn't refresh unless doors were opened. Don't remember how long that lasted but I do
remember the relief I felt one day when logging on to refresh them all and found them not
needing it.
One of my greatest wishes for UO would be to allow L-shaped homes to be squared off IF
allowed by placement rules and made customize-able without changing grandfathered status.
Then I could have my museum on Atl instead of on another shard where I have a larger
customized home. Sadly, there are not enough players in this situation to ever have a loud
enough voice for the powers that be to care.