There are two things to consider when setting up an extended network with multiple access points:
1. Placing the APs where the users will have a good wifi signal over the entire intended coverage area.
2. "Backhauling" the data from each AP to the house's cable / DSL / etc. modem and thus ultimately to the Internet.
A simple wifi extender like the WD3000RP uses the same wifi radio for the users and the backhaul. This means that it needs a wifi link to the main router for (2) to work. On the other hand, for condition (1) you'd like to install it where there is no wifi signal from the main router. Ultimately you compromise and put it at a mid-way point.
The best backhaul by far is an Ethernet cable. If the house is fairly new it may have some cat 5 / 6 cables installed during construction. Or it would be relatively easy to run them through the attic or basement.
There are units mentioned in this thread that look like the WD3000RP but backhaul through the power wiring. This sort of system requires another Ethernet to power line converter that you would install at the modem and connect to it with a short Ethernet cable. Often this device is included in the box as a kit. The power line backhaul does not require a wifi path from the main router to the extender. That lets you place it basically anywhere there is an outlet connected to the same power system as the one at the main router.
Power line Ethernet can be hit or miss, depending on the wiring and other devices in the house, and whether you can get the two devices on the same "leg" of the power supply. But it has gotten better.