Export Buildings
From the Townbuilder
If you have the building loaded in the Townbuilder, press File > Download in the top menu. You will be asked to save your building beforehand. Sign in first. Guests can build, but download needs an account.
From your saved buildings
If you want to export a previously saved building, open your saved buildings. There, hover over the building you want to download and click on the download button.
Either way, the building is added to the download queue in the top bar. The building will automatically download as a zip file once the process is finished (unless you disable this in your settings).
Unlock required
You can build with every model, including packs you have not unlocked. Download is allowed only when every asset pack used in the building is unlocked on the signed-in account.
If a pack is missing, a dialog lists it and offers unlock. Saving is still allowed while download is blocked, so you can unlock the packs later and download then.
DO NOT CLOSE YOUR BROWSER TAB OR ANY UNFINISHED EXPORTS WILL BE CANCELED. As all exports are currently done locally on your machine, closing the browser tab while exporting will cancel the process.