At a minimum, the underside of the appendage should have the same level of protection as the siding, so the same siding material could be used. Alternatively, you could use a noncombustible material (e.g., a fiber cement panel), or ignition-resistant material (e.g., exterior fire-retardant treated plywood). If an open-frame design is desired, exterior fire-retardant treated wood, or larger (oversized) wood members should be used. Don't forget to maintain vegetation, and avoid storing combustible materials under the appendage.