Programs I use:
Inside UO
Micrografx Paint

Inside UO views the graphics files in the UO folder, it does not interfere with anything in the client. With it I am able to copy and paste the items I need to make the renders. Yes, it is tedious to do one tile at a time, but I think it will save alot of time when the tool comes out. It should eliminate unnecessary editing, and save time if you already have a general idea about what you feel your home should look like. One of my suggestions is to take some time and think about what functions you want in your design, what you need, what you would like. Then sit down and start designing!

Micrografx Paint, paint shop pro or photoshop have the same or better capabilities, is the actual program I do the renders in. I Mask items and place them as the tool would. I have 5 or 6 working files so I don't have to mask everything each time I do a new render. It is rather satisfying when the whole house is finished.

But before you start designing your own home, you need to be familiar with tiles, spaces, stories, and what makes them usable, as far as decorating goes, so that you do not end up creating a wonderful house you can't fit anything in. Remember, plots will keep the same stories they have now. That translates to: 2 story plots will remain only 2 story plots, 1 story - 1 story, and 3 story -3 story. The difference is, those houses that currently have a pitched roof will get a flat walkable roof. Not all designs will benefit from this, but many will. Those that currently have a walkable roof will keep that roof, but those with a pitched roof like the villa, brick, small, log cabin, and the like will get a flat roof. As Sannio stated, when you customize a home, they will all be either flat walkable roofs, or no roofs at all. It is up to you what to do with the roof. Pitched roofs are not coded into the tool. I will discuss what makes tiles unusable at the end of the list.

The three different sizes for each house translate to: what you need clear to place the type footprint when you are actually placing the house, actual walls of the building itself/foundation, and usable tiles within the foundation if you have walls around the whole house. If for instance you chose to take a small house and have nothing but a floor over the entire foundation, then you'd actually get the 6x6 usable tile print. The walls extend out one tile from the right (east) and bottom (south) that are not usable inside the home. So whenever you place an east or south wall, count the tile directly north (for south walls) or west (for east walls), unusable. The only exception to this is the small wall pieces. They can be used for walkable/non walkable items. Yet they can not be counted for things like add-ons, stone tables, and such. So, when adding walls be aware of this, each south or east wall added takes away usable tiles the length of the wall placed.