Generally, a landowner can build a fence as long as it is on their side of the property line. However, depending on where you live, there may be local bylaws that regulate how close a fence can be built to buildings on the same lot or neighbouring lots. As well, local bylaws often contain limitations on the placement and height of fences in order to address safety hazards.
You may want to call your municipality's building department to find out what these restrictions and rules are in your area. You could also search the website CivicInfo to find local bylaws relating to fences in your community. In the search box, you can try typing in “fence bylaw [community name]”.
If there are no such restrictions, and you are still concerned, you can try telling your neighbour what your concerns are. If they're still not receptive, there are other actions you might be able to take. You may find our page on fences and neighbours helpful. Refer to the "Deal with the problem" section for steps you can take when you have an issue with your neighbour's proposal for a fence.