If your house came with a deck that's simply not big enough to accommodate your gatherings, you have two choices: You can tear out the old one and build a new structure or you can attach an extension. Here we address attaching an extension, which is the simpler solution of the two.
A deck extension is not difficult to build. Essentially it is an added section supported by part of the existing structure. The new section can either be a step up or a step down. Another possibility is to restyle a slab patio as a deck. In that case, if the concrete is not powdery or breaking into chunks, you can install sleepers and fasten down decking in a weekend. Be sure you have a deck plan before you get started.
Decking laid over concrete must be raised from the surface of the concrete with 2x4 pressure-treated sleepers. Snap chalk lines 16 inches on center on the surface of the slab to mark sleeper locations. Install the 2x4s flat. Fasten them with masonry nails or a powder-actuated tool. Cut and install the decking. Consider patterns other than parallel decking—the flat 2x4s provide a sufficient nailing surface for many patterns.
To frame an extension that rises one step from the existing deck, lay out the post locations and cut the posts so the joists rest on the surface of the old deck. Build the frame and toenail it to the old deck. Design your extension to use 2x6s, if possible. That way you'll have about a 7-inch step to the new deck.
To frame a step-down deck extension, treat the old beam as a ledger for the new frame. Mark the joist locations on the beam and lay out the site with batterboards and mason's lines. Hang the joists in joist hangers.