Because ABC = 90, ACB = 45, ADB = 30,

So AB=BC= 1/2AD,

Let the building height be h,

That is AB=BC=H,

AD=2H，

BD=BC+CD=H+60

From Pythagorean theorem, in △ABD, AB? +BD？ =AD？

h？ +(H+60)？ =(2H)？

Solve this equation to get H=60/(√3- 1).