I don't understand a thing... You have a 300 square meters house, but you can't afford 3-4 routers to cover the whole area... Man, a new router is just under 20 dollars here (D-Link like mine) and will cover 40-50 meters without any problems. You can go even for a N router which can cover more but are a little more expensive.

You can also try a single router connected to some Wi-Fi AP's (wireless repeater/switch) and gain the same effect.

P.S. Even if you put the Wi-Fi on top of your house the walls variable is still in talk (so in some rooms the signal will be better in others not so good).

P.S.S. Some routers are better than others @signal strength. Example: I live at the lowest floor in apartment building and me, and the neighbor from the 2nd floor both have Wi-Fi routers. I can see his router, but he can't see mine, why because mine is a 5 year old model with only one antenna and his router is a N router with 3 antennas.

P.S.S.S How I recommend to setup a multi-router network. Install 1st router in the most central spot of your house. Take a laptop/phone with a signal strength monitor and move away. Move away until you have only 2 lines (out of 5) of signal strength left. Stop there and measure the distance you walked. Now the same distance, walk it again but from the sign (where you have 2 signal bars) to which ever direction you want right now. Install 2nd router. Repeat until your whole house is covered.

When configuring the routers, set the admin password to 192.168.0.X where X is the number of your router. The most central should have X=1 and the internet connection, the rest should have whatever administration IP but the same SSID as the 1st.

That's about it, now you have a multi-router home network, with individual router configuration (you can configure each and every router - if you do the admin ip step) for just under 100$, or 200 if you want state of the art routers .