Use graph paper to create a layout of your home. Go over several choices to locate the best area to expand into a living room. Consider taking down a large section of exterior wall to open the double wide extensively. It's important to talk with mobile home manufacturers, if possible, to get ideas. All phases of taking out part of an exterior wall is not necessarily a DIY project. Keep in mind that supporting metal frame work with a new, reinforced metal framing may be required.
Sketch the exterior of the double wide with the addition. You might build a 20-foot-square addition, for example, on a foundation of concrete blocks covered by stucco. Draw the new living room as a four-season room with lots of windows as one option, or stacked stone or house siding made of concrete fiber can work well, too.
Use an existing sliding door doorway for an entrance. Open a wall as one choice, but look at how to access your new living room via a pre-made doorway. Take out an existing sliding glass door system to build an entryway to the new space. This is a more cost-effective way to stay within budget, because removing a large section of exterior wall may call for extensive labor assistance from others.
Roofing issues are important to planning the living room. It's always good to build a standard house roof over a double wide. The cost is significant, but it's easier to construct a living room addition to an existing stick-built roof. Other alternatives include constructing a living room that adjoins the mobile home via a higher living room roof. Adding flashing and sealants to common wall and roof areas will keep weather out.
Foundation issues will need attention before construction begins. New duct work and venting for heating and cooling the room come into play. New wiring for outlets and light fixtures may need wiring routed through the crawlspace. Heavy insulation in the crawlspace will help, too. You may want to build an access door on the foundation wall of the new living room.