If this fixes the issue but you still want to get rid of the walls & floors replace them with ones you do like in BUILD mode, then use the in-game trash can to delete the files you don't want. This should prevent the problem. Unfortunately you may have to live with the terrain paints since detaching those from houses that use them is next to impossible!

If you're certain that isn't the issue then follow the steps below to get rid of the problem Lot.

First you need to identify the neighbourhood NUMBER the lot is in. Neighbourhoods will be numbered in the same order as they appear in your Neighbourhood Selection screen - the first Neighbourhood on the left hand side will be N001, the second N002, third N003 and so on. Load your game up and count from left to right along the Neighbourhood Selector until you've found the Neighbourhood that your problem house is in.

Exit the game and open up your My Documents/EA Games/The Sims 2/Neighborhoods folder and locate the corresponding N00X folder.

In it you will find a selection of N00X_LotX.package files - each one corresponds to a house placed in the neighbourhood. If you have recently placed the house then sort the folder by modified and change the folder view to show details. Looking at the date and time the files were created, find the house that has the approximate date and time of when you placed the house in the neighbourhood.

Once you are certain you have found the problem house simply delete the file. This will remove the instance of it from your Neighbourhood.

Next we need to clear the house from the Lots & Houses Bin. Open the Lot Catalog folder:-

Again, sort by modified and show details will help you identify what was added when - if you know the date and approximate time you installed the house you can find the file that matches that and simply delete it.

First you need to download and install SimPE - SimPE 'Light' Download is just fine for this purpose. Follow the on-screen instructions to install it. Once installed open the appropriate neighbourhood folder as described in method 2 above.

Pick one of the files in the appropriate Neighborhoods/N00X/Lots folder you think may be a possible candidate for your busted house just double click if you have your package files now associated with SimPE or if not right click on the file and choose Open With - SimPE. Once the program has loaded you'll see 3 different panels, the top 2 smaller windows should have big lists of stuff.

In the LEFT TOP window (Resource Tree) look for the entries marked 'jpg/tga/png Image (IMG)' - these are the previews of the lot

In the RIGHT TOP window select each entry in turn by clicking on it. You should see a photo appears in the bottom Plugin View window. These are the images the Exchange uses for generating house previews and each shows a different floor walls up/down view of the lot.

If its not the house you're looking for then open another and another until you find the one you're after. As soon as you've found the one that doesn't work close SimPE and delete that package file from the folder. Repeat this process with your Lot Catalog folder to remove the broken lot from your Lots & Houses Bin