Some types of building construction use wall panels with foil backs (for thermal insulation) which is dreadful for Wi-Fi.
There's an FAQ about diagnosing Wi-Fi issues in pinned on the Networking and NAS forum at AVF which, amongst other things, talks about checking for co-channel interference (from the neighbours) and other horrors.
One thing you could do to try and assess culpability of your router is to try a different Wi-Fi router/AP and see how that compares (it doesn't need to be connected to the Internet, it just needs to be advertising an SSID so you can see how it compares using a tool such as Acrylic Wi-Fi Scanner.)
At a pinch you could perhaps use a smart phone if it's one with a "personal hotspot" function like a iPhone. However, they have piddly little antenna and run of batteries, so probably aren't great to start with - still you could set it up and see how it compares in both the "good" and "bandit" locations.
Not all powerline adapters have Wi-Fi Access Points built in, but some do. There's an FAQ about "HomePlugs" pinned in the Networking and NAS forum also.