The best time to incorporate solar power into the design of a home or building is in the initial construction. Windows are best facing south to let maximum sunlight (hence greenhouse heat) in during the winter. Likewise, avoid windows facing directly into the summertime sun to keep things cooler, so less power is needed for fans or air conditioning. The maximum roof space, however, should always get sunlight exposure.
Carefully consider the types of solar panels you plan on having installed. Long ago, if you didn't have enough roof space, you'd need very expensive high efficiency mono-crystalline panels. There are now more advanced polycrystalline panels and even thin film panels. If you've got enough roof space, you might wish to look at the size of the panels in relation to their output capacity to avoid having insufficient panels that you need more of later.
Leaky solar system, enovative group investigates…
Before installing any form of solar panels, be familiar with local rules and regulations. There are lots of places where some permits are required. The last thing you want is to have the system put it and then you are forced to remove it later.