Do you need a cable or is a good Wi-Fi connection enough? You have three options, since 26m is not very far.
- Run fiber. You can use so called media converted to convert from ethernet (copper) to fiber, you need them on both ends, they both need power.
- Run an ethernet cable. Since it runs outside, make sure you use a S/FTP cable that is properly grounded on both ends and use ethernet surge protectors to prevent your gear from being fried by lightning.
- Install a directional Wi-Fi access point on the outside of your house pointing in the direction of your garden office.