re: What's the proper etiquette here?

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On the property line. From the ground, straight up into the sky, that's your property. You can shave off an entire side if it encroaches on your property. Legally.

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this. But as a courtesy, I'd speak to the neighbor first and let him know your intentions. In the end, you're going to do what you want to, so no reason to appear an a-hole about it.

However, think long about getting rid of the shade that tree provides. You may regret not having it in the future. Especially if pruning the tree ends up harming it.