Prefab, modular, and panelized are partially or mostly built in a factory, then shipped to your site and assembled or placed on a foundation. Some companies provide a “turnkey” service that helps with permitting and all on-site construction, from laying the foundation to utility hookups. If not, you’ll need to hire additional professionals to help.
Using a prefab design can save time and make clearer what your completed project will look and feel like, but they also typically offer less customization, can be difficult to permit and finance, and often cost the same as site-built ADUs. People are often surprised at the cost, but transportation, crane, foundation, and sales tax can end up being 20-40% of the total cost. Some companies do offer a smaller price point.
To explore prefab or modular ADU options, you will want to get in touch with companies directly. They will visit, evaluate your property, and make recommendations. You can find prefab contractors by researching online.