A neighbor intends to construct an ADU that will completely block my ocean view and invade my privacy by inviting residents to congregate directly in front of my living room windows. Also, at 16 feet in height it will cast a shadow on my existing solar panels. Can he do that?
It depends on the location and the nature of the structure. In Seattle, for example, it requires a permit. Usually, any such structure does require a building permit, which means that the government has to approve the plans w.r.t. offset requirement, height requirements and so on. There may be a view ordinance, or not; you may have a view easement, or not. Whatever the case may be, you should not assume that the government agency in charge will vigorously work to protect your interests over the neighbor's interest. You own attorney is the one who will vigorously and professionally defend exclusively your interests (likewise, the neighbor's attorney). Your description doesn't explain how this would "invite residents to congregate directly in front of my living room windows", which seems unlikely for a dwelling. If for instance this is really a bar and not a dwelling, then zoning issues about businesses arise.
Providing that the development complied with local planning requirements under the Environmental Planning and Assessment Act 1979, your neighbor can do what they like.
Very occasionally there may be a covenant over the land that protects the view. It’s very rare but can happen where the blocks were part of the same subdivision and the developer decided to protect the view for some reason - often because they were keeping the land so protected.
The owner of a property can do as he wishes to his property. Provided that it doesnt violate zoning code, isnt subject to an HOA, and the activity itself isnt inherently criminal.
In some states you dont even have to get a building permit or even so much as adhere to construction code... provided its construction isnt a risk to neighbors or utilities, isnt a violation of municipal law, and provided that the owner never intends to sell the property.
A neighbor does not own the airspace above someone elses property (property owner himself may not even), nor do they own the view through someone elses property (arguably a privacy violation in its own right).