You can attach a deck extension to your existing structure.
If your existing deck is a couple of decades old and looking worn and small, you may reach a decision that it’s time to go for Deck Version 2.0. This can entail demolishing the original deck and starting over, or adding an extension to increase its size. As with the original construction of your deck, planning is nine-tenths of the battle.
Inspect the substructure of the deck -- the posts, beams and joists -- looking for splits in the wood, or sagging or leaning members that indicates insufficient engineering and support in the original construction, which may not have been to code. If you find problems, your best avenue is to remove both the deck boards and the substructure and start over. If the substructure is sound, you can keep it in place and add a new section or sections to it.
Consult your local planning department on its requirements for permits for deck additions or replacements. You may be required to provide plans for the work with a stamp from a state-certified engineer or architect. Even if these are not required, it’s wisest to consult an engineer or architect, who will know the span guidelines for your planned joists and beams, as well as any recommendations, such as seismic ties for earthquake-prone areas.
Sketch a planned larger deck on graph paper, showing a side or “elevation” view and an overhead view, if you are designing a deck extension. Carefully note existing beams, joists and posts, railings and stairways. Make several copies to allow you to tweak your designs. Work with deck-planning software, available free online, to design a complete replacement.
Designate on your plans the end or rim joists -- which run perpendicular to your house or parallel at the far end of the existing deck, respectively -- that can serve as junctions to a step-up or a step-down platform addition, or even a same-level addition. Draw a rectangle representing the addition, such that it shares a joist with the current deck. Add locations for support posts, indicated by circles noted on the overhead view, confirming these locations with your architect or engineer.
Follow your plans to install your deck addition, pouring concrete supports, installing posts on them, and topping them with a beam of doubled or tripled lumber, typically 2-by-10s. Run your regular joists, typically 2-by-6s, atop the beam so their ends touch the top of the existing deck platform, creating a step-up new platform. Add an end joist to each end of the new joists. Run the joists atop the beam and into joist hangers on your existing joist or beam to create a step-down platform.
If you decide to demo your existing deck, remove its parts in top-down order: Take off the top rails and balusters with a pry bar, then the deck boards, joists and beams. You can avoid dumpster and haulage charges by pulling the nails out with a nail puller and selling the lumber for a nominal price as salvage material on Craigslist or local listserves, where potential purchasers can repurpose it for rustic greenhouses, benches and temporary walls.
Parrish, Rogue. "How to Increase the Size of an Existing Deck." Home Guides | SF Gate, http://homeguides.sfgate.com/increase-size-existing-deck-30156.html. Accessed 22 April 2019.