Bear with me through this post; It is possible that she was "trained" not to go wee in the previous owners yard, purposely or accidently. Some houseing developments ask that your dog only go in a specific spot, or the owners might have not reinforced that she go out in the garden. You can train her to go out there; whenever she does make sure that you have treats and you make her feel like the smartest dog in the world for weeing in the yard. Make sure they are treats that she really wants and so eventually when you let her out she will throw you behaviours like sit, and if that doesn't work she will try other things. Make sure you treat her when she goes every time until she goes more frequently.
If she really likes being in the yard, then make sure that she can muck about a bit before you bring her in, otherwise the dog thinks "hmmm i like it out here but whenever i wee, Mum brings me back in. I am going to hold it and play a bit longer"
You could also add a urination cue. My cocker used to wee whenever we said " be a good girl!" My dogs now go when you say "peepee on the grass" If she only goes once a day that could be tricky, but worth a try.
She should go about every 8 hours, so 3 times a day ideally. I would caution against forceing more water in at the start if this is a behaviour issue. You are certain that there is no undiscovered toilet in your house? Just make sure that there is water available for her at all times. Good luck.