You must check the zoning first to make sure it is possible. An architect will look at a survey and the zoning map and advise you if it is possible and how many squaree feet you could add and what other requirements you will need to meet the current building requirement. He/she will also assist you to design the space. After you know exactly what you want to build and the quality of the construction then you can speak to three different licenced contractors to get estimates. It could cost you between $175 to $250 per square feet to build. You will hire an architect; he will do the plans; submit it to the building dept. for approval. Once you get the approval, you give it to your contractors who has to provide their licence and insurance and apply for a permit at the building dept.. After they obtain a permit then they can start the job. You will need several inspections including electrical and plumming and construction etc. before you can obtain final certificate of occupancy.In the meantime if anything goes wrong you will be ultimately responsible for violations and fines caused by your contractors. It may cost you less to find the right two family and buy it instead. There is a lot more involve in the process. This process is for the Bronx. Seek more expert advise.