Ok here goes, I'll try and keep it simple.
You will need a few programs;
I recomend UOX3 V0.70.03 for your Development World EMU.
I recomend Version 2.0 of the Client.exe file for UO
You will need InsideUO (program gives you all the Hexcodes for Items)
Ok, once you have UOX3 setup and running as the Dev Server.
Login to the game with the 2.0 client.
Run InsideUO
When you are standing at the spot that you wish to make a house, use InsideUO and click on the Artwork look for the type of wall that you would like to use for the house, once you have found the wall type you want to use "single click on the image" you will see the section "Modle No." give a set of numbers similar to this; 77 (0x4D).
The number you want is the 0x4D, so now return to the Client and type the following. 'add 00 4D [enter]
then target the spot on the ground where you want the wall to appear and it will be placed there.
now to speed the development process up a bit, you can use this statement to create an entire wall; 'tile 00 4D [enter]
and then click on the ground at the starting spot of the wall, then click on the ending spot of the wall, and a wall will apear.
Just keep locating the wall type you want and then use the 'TILE command to place it. Once you have all 4 walls built, just 'REMOVE 1 peice of the wall where you want a door.
For windows, simply find the window type you want in INSIDEUO and get the Hex# (ie: 59 (0x3B) ) and in the client use 'SETID 00 3b [enter]
then click on the peice of existing wall that you want to change to a window.
Adding a Roof is alot more complicated but its done using the same kind of commands and methods. You would be best to check the Tutorial that was just recently posted on Ryandor's Site.
I hope this helps, and I tried not to get too complicated.
Bottom line is you just need to practice, and get used to the commands and how they work.
Regards
-Imprint