Check whether you are legally able to build a modular addition. Review your deed paperwork to ensure you have the legal right and look up the zoning laws where you currently live. Consult a lawyer if you have questions about this process.
Get your home appraised. Apply for an equity loan to cover the cost of the addition.
Visit the dealer from whom you bought your original modular home and request a ballpark price for the addition. If you don't feel comfortable with the dealer after your meeting, don't buy yet. Shop around. Visit several dealers until you find one with whom you feel comfortable and who quotes an affordable price.
Give photographs of your existing home to the dealer to help her design your addition. Invite the dealer to your home so she can take measurements and come up with design ideas.
Purchase the design from your dealer once it is complete. You may need to use loan funds to do this. After you purchase the design, hire a contractor to build the addition. You will have to pay the contractor separately for his work.