How to join the horizontal slat fence with the neighbor’s fence (vertical fence)

fence

I am thinking about building a horizontal fence to close the open space left by the previous owner to the left of my garage.
How do I join my fence with the exiting fence of my neighbor. Do I have to build my own post next to the one behind the blue garbage bin?

Edit: I am not worried about the legal implications and it does not matter on whose property is the fence. I am just looking for a technical solution

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The picture below shows the intended functionality
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Update: I looked at that post and it was set in concrete and it is rotten as it is the on to the left side on my neighbor's property. There is another one rotten just behind the small tree at the end of the area that I am trying to cover There is no way that I am going to attach my gate to that post unless I decide to replace it and to split the costs with my neighbor. However doing that will complicate the solution. I am planning on using one of these:
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for the removable central post. I would like to use the same for all the posts but I am not sure if this is advisable. This will be a horizontal fence, made of cedar boards cut of 1x(2" or 3" or 4" or 6")x 8" to match my needs and to build the design that I am planning to use

Best Answer

Before you start, talk to your neighbor first, get a feel for any concerns if any they have. Then inspect the post to see if it would hold a section of fence. Then go to it.

EDIT: Ok lets try a different approach, dose your property line split the post, (red line), or meets it (green line)?

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This will make a difference. I would also consider placing a stationary panel first because the swing of you gate would be hitting the curb on the driveway.