Most experts agree that a deck extension project is no easy task, and that it's best to hire an experienced contractor to do the work for you. Make sure the contractor is licensed, has insurance and will obtain any necessary permits (if the deck is close enough to the ground, it may not necessitate a permit). Shop around for a good estimate, giving each contractor specific details on what you have in mind.
Even if you hire a contractor, it's wise to know the basic deck-building process so you understand what's going on. Familiarize yourself with the names of deck components. The ledger is the board that affixes the deck to the house. Vertical posts are set deep into the ground on concrete and secured with gravel. Local codes will likely dictate the minimum depth of post holes for proper support, which will probably be below the frost line. You'll probably need to dig post holes and install new posts for additional support for your extension. You can mix the concrete for the posts on site or get it pre-mixed from a local supplier.
Posts attach to horizontal beams (also called girders). The joists are the horizontal support boards that sit underneath and perpendicular to the surface decking. Codes may also specify the distance between joists. When extending your deck, it's especially important to know the safety rules and regulations about cantilevering. Cantilevering refers to a beam that extends beyond a support post or a joist that extends beyond a beam. The further it extends, the less safe it will be. For proper drainage, make sure the surface decking boards are slightly spaced (about the width of a 16d nail) [source: Lowes].
If the deck extension is far from the ground, safety considerations also include railing posts and balusters (vertical support pillars between posts). And if you want access to the lawn, you'll also need to think about stairs. Codes may dictate the requirements for proper rise (height) and run (depth) of each step.
Overall, the project will take several days (or several weekends) of tough, meticulous work, and costs can reach upwards of $5,000. But in the end, you'll have a lovely outdoor entertaining and living space that could last decades.
For lots more information on construction projects, see the links on the next page.