|By default, most SOHO routers create a DHCP scope of the following IP's: 100 to 199|
Check the DHCP scope on the router in the house. Write down it's scope. Create a DHCP scope on your router in your flat using IP's not in the scope of the DHCP server in the house. (as per my example above, you would use say, 200 to 250, or 50 to 99 in your router in your flat)
This would prevent overlap and possible duplicate IP's and allow you to do what you want while still being able to access the internet via the router in the house.