Decide what items are essential to have in the trailer. Assume that the building estimate is overly optimistic and prepare to be living in the trailer for more time than the builder says that it will take to build the house. There should be enough furniture to ensure that every family member will have a place to sleep, sit for meals and relax. Other than that, each family must decide what is most important to them, such as a television, bookshelf, computer area. Maintaining an uncrowded atmosphere is important even while trying to organize the space to accommodate the necessary items.
Find a storage facility for the household items that cannot fit into the trailer. Be creative in finding extra storage space nearby--an old fridge or box placed in a dug-out hole under the trailer can be helpful. Likewise, an old car can be put on cinderblocks next to the trailer and used for storage.
Keep the most essential items in the trailer and put as much as possible in storage. Clothes for off-season can be kept stored and the clothes switched when necessary. Multiple sets of dishes don't need to be kept in the kitchen....guests will understand if they are served on the "everyday" dishes while the family lives in a trailer. Likewise, it is not a problem to live for a year without a pasta-maker or a bread machiner.
When the family moves into their new house, it may be decided that so many of those "important" items that were so necessary to everyday life are, in fact, not necessary, and the new house might end up being much less cluttered.
Make sure that all local laws regarding utility hookups are adhered to. These laws can be researched through the local municipality. Complete the hookup technicalities for electricity, water and gas before moving into the trailer. The building contractor might be able to arrange this as part of the building process and the hookups can be used afterward as part of the house plan.
Organize the interior of the trailer as efficiently as possible. Every item should have a space where it can be stored. Don't allow for any clutter or disorganization.
Make sure there is plenty of shelf space that will facilitate this organization. Shelves can easily be hung on the walls of the trailer and don't need to take up much space, but having them available will make it much easier to keep everything organized and in its place. Train all family members to put things away.
Schedule much of the day away from the trailer. Eat outdoors whenever possible even if it's at a picnic table next to the trailer. Schedule afternoons in the library or at sports activities. Visit friends and relatives frequently.
Keep the amount of time when the whole family is in the small trailer space together as little as possible.