Hye there..
Of course can..
Normally owners want to do like that..
But this works must be detail carefull because its detached at existing structure..

Agree with kelvyn..get quotations first then only decide..for general quotation u can list down what do u want to do and maybe attach with the floor plan..and u can get price expectations before u know the actual price after site visit..must be see the actual site to get details quotation..