Here is the information you'd have to consider:
1. Find out if there is any room on the property to building additional structures. Before, you'd have to go to the city to find out. Now, you can just relax with a glass of wine, stay home and get the CityStructure report for your address on your computer. CityStructure report shows you the maximum lot coverage area and the Gross Floor Areas of the addition. This includes: covered decks, porches, landings and stairs, any deck on the roof of a building or accessory structure or over an enclosed space and paved ground area.
2. Design considerations: Now that you found out the magic area you may be allowed to build for your addition, see if you can fit any usable spaces in it. Important to know is that additions less than 600 square feet or no more than 15% of the lot area, which ever is less, are permitted with a Zoning Certificate. Just make sure you stay within the zoning boundaries (related to setback from property lines, parking, etc.) as shown in the CityStructure report.
Need ideas of the spaces size you need? See below some diagrams with suggestions. Select the spaces you may need and add the corresponding sizes to approximate the total new area you'd need. Make sure the spaces of the existing house are connected with the proposed spaces.
3. Addition Height: the average height of the proposed addition may not exceed 14 – 16 ft, depending on the Zoning District. If the structure is on a slopped lot, the way you measure is different. Not you worry, CityStructure report takes the slope of the site into consideration.
4. Demolition: keep the demolition area under 50% of the existing exterior wall framing AND under 50% of the roof framing
5. Clearance: the new addition has to keep at least 3 ft clear from existing structures on the lot
Bottom line, try to avoid falling into a Major Residential Addition project if not necessary and keep it just Minor Residential Addition in order get your Zoning Certificate ...and start digging!