While there he began planning the design and construction of a suitable mausoleum , a kind of tomb , and a garden in Agra for his wife , a task that would take more than 20 years to complete , the Taj Mahal . While there , he began planning the design and construction of a suitable mausoleum and funerary garden in Agra for his wife . It was a task that would take more than 22 years to complete : the Taj Mahal .