Before you decide to add onto an existing deck, you should talk to your local building department. This may not be allowed in your area for a number of reasons. You may want to bring a few pictures of the existing deck along to help explain your situation. The inspector will usually want to review the permit and inspection records for the existing deck and may call for an as-built framing inspection to verify the condition of the deck before issuing a permit.
Most decks are not built with a future addition or extension in mind. Because of this, your frost footings are probably not large enough to support the new loads created by the addition. We recommend framing the new deck addition independently of the existing frame. In order to do this, you will need to install new beams and footings to support the addition.
Some deck builders choose to sister the new joists back over the existing beam and install a new beam in front of the existing deck. The sistered joists are nailed into the sides of the existing joists and extend past the beam at least 6". Hurricane ties are used to create a positive joist-to-beam connection. It may be necessary to install new posts and footings below the existing deck beam to support the additional loads. Most people that add onto their decks also replace the decking and railings on the existing deck at the same time. This will create a more seamless addition without noticeable color and weathering differences in materials.