Most fencing regulations limit the height of artificial fences in residential areas to four feet in the front yard and six feet in the rear yard.
The neighbour who constructed the fence is exclusively responsible for its upkeep unless the other neighbour decides to utilise it. Every state defines "use" differently (see above), but most regulations are satisfied when a property owner uses an existing fence to enclose their property. In many states, fencing laws require the neighbour to pay half of the fence's value to the other proprietor.
It is imperative to inform the neighbour of the violation, as they may need to be made aware of the ordinance. If the neighbour refuses to comply, report the violation to the city. The municipality will notify the neighbour in writing and request compliance. If the neighbour refuses to comply, the city may impose a fine and file a lawsuit to compel compliance. You may prosecute your neighbour in small claims court for the loss of enjoyment of your property, but this will not result in the fence's removal. A trial court judge can issue an injunction if you want the neighbour to remove the wall.
Yes, you may construct it near your neighbour's fence. You are in the clear as long as you remain within your property line. One consideration is that your fence posts must be staggered relative to the existing ones.