Getting internet in my Garden office

When you're looking to build, the biggest thing I recommend to people, is to check the type of wall they're going to be using, as well as the insulation (this includes various forms of vapor barriers).

The main thing you have to ask, is whether or not you're planning on using any foil lined medium for those applications. If you're using foil backed insulation, any signal would be greatly reduced at any range, but heavily so at 100ft.

The next question is, is it direct line of sight, including any overgrowth of foliage? They have very little impact on wifi signal, but there IS some impact.

When in doubt, you can always run a simple site survey, yourself, if you have access to an Android based device. (Phone or tablet generally work....Apple killed off the application I usually use for this, awhile ago, so I stopped recommending iOS devices for this)

There are many applications you can use to test the signal strength, but I prefer "Wifi Analyzer" in the Google Play Store, here:
https://play.google.com/store/apps/details?id=com.farproc.wifi.analyzer&hl=en

When you launch the application, you can swipe left and right through the various screens, but in all, you'll want to see if your current WiFi extends that far. As long as it doesn't dip down, below -70 or so, you SHOULD be able to have at least some connectivity out at the office.

At that point, you can look into a Wifi Range extender, which is a wireless device that sort of piggy backs onto your existing wireless device, to make the range larger. These extenders often operate as a 'Repeater' or a 'Bridge', and can be tied into other devices either wirelessly or hardwired.

The first thing, however, is to check that signal strength. You may not need to buy anything more than you already have. :)