you can put the house down on a plot and then click on it so you get the white info box in the botton left corner of that box to can click to package lot either to file or to the main sims site
if you package to file it will be saved in
My documents/EA games/the Sims 2/PackagedLots
If you're planning to package your lots for uploading and sharing with others, you should strictly avoid packaging lots directly to the Exchange on the Sims 2 official website if you have used any custom content whatsoever. Most content creators specifically ask that you do not upload their work to the Exchange. Also, make sure that you've read the policies of each content creator carefully before including custom content in your lot.
You will want to check the lot package with the Clean Installer before uploading, to make sure that it hasn't included custom content you didn't intend to include.
And be sure to credit all custom content authors in your upload! This is extremely important.
Of course, if you're just trying to package a lot for your own use, then you don't have to worry about all of the above.