If at all possible you would be best to run an Ethernet cable from your house to the summer house/office. You can then connect a wireless access point to it. It will give you a far better signal than using wireless extenders/boosters.
If the summer house is on the same electrical consumer unit (fuse box) at the main house you could try using powerline adaptors. This takes the broadband signal from you house along the electrical cables to the summer house where you can have a wireless powerline adaptor broadcasting the signal for your wireless devices.
