Light in Winter and Summer
In summer, unless you have sited your garden greenhouse in dense shade there is likely to be sufficient light for plants to grow successfully. The problem of lack of light is most likely to occur in winter and spring when the sun is low in the sky. 

Unfortunately, most garden greenhouses are bought and erected in the summer months in preparation for the following spring. The shading effect of trees, hedges and other structures may appear minimal in the summer but when winter and spring come along, they may reduce the amount of available light considerably.

In late spring (April), the same object will  cast a very much reduced shadow which is just a bit more than its own height. This shadow is further reduced up till mid-summer. The diagram on the right illustrates the shade effect. 

The diagram on the left illustrates the shade effect for a variety of different heights in April whilst the diagram below illustrates the massively increased amount of shade cast in December and January.

So, if you are building a greenhouse in summer get your tape measure out and measure carefully!