You will possibly need both an Architects and an Engineers certifcate. The reason being is that the conversion needs to comply with building regulations. The engineers cert will give you certification on the struture and is imperitive if the structure is changed as this will be requested if you wish to sell in the future. The Architects cert is needed to deal with general building regs, i.e the width, height of access complies. Also the Arch can certify for fire complience. However the engineer maybe able to certify for all, however the architect will only be able to certify for non structural items.