I have a elevated deck (~8ft) attached to the back of my home, and I would like to add about 5 feet to it. What is the proper way to add this extension?
I want to extend further away from the home. For example:
Home -> DECK -> Extend This Way
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Depending very much on your Local Authority Having Jurisdiction, you might or might not need permits, plan approval, etc. and in most cases getting that part sorted (if required) will automatically force you to get competent advice on the next bit...
Which is how you hold it up - that depends on the shape of the land, the type of soils or bare rock or whatever you have under the proposed area of the new deck, the height of the deck, the local snow, wind, and seismic loadings required, frost depth if applicable, as well as design requirements for the live load of people being on the deck, and how the current deck is held up.
Doubling the size of the deck will be enough of a job that you should look very, very carefully at the state of the current deck to determine if remove and replace actually makes more sense than preserving the old deck and extend with 50% new.
The most common will be new pressure-treated posts into the ground or connected to a concrete footer for the extension and probably posts and a new rim joist added to the existing structure. That way you aren't really adding stress to the old structure. 4x4 if low, like right off the ground, 6x6 if in the air, 8x8 if up past six feet. The rest is just boards to join the posts and hold the deck boards.
I leave to you permitting, etc. if required.