First, no appraiser should do an assignment in the City with out StreetEasy, this tool is as important as MLS in any area, and is one of my most important tools when appraising in the big city....
In Tribeca the values vary building by building, if you PM me the address I can try to help you, outdoor space is a very important factor....
Check this link for sales in the area, but you have to figure out in which category your subject falls into, there is a vast amount of sales, and you'll have to figure out how desirable your condo project is, if it's one of the bigger buildings, then your job is a bit easier, the smaller ones are much more difficult to figure out, if there are any sales in the subject project they will give an idea of the price per Sq. Ft. this building is going for, now try to find a similar size building that has a similar sale with outdoor space and the units without the outdoor space are selling in harmony with the sales in your building.
http://www.streeteasy.com/nyc/compa...&subject_listing_id=227976&listing_class=Sale
For example here is a sale in 200 Chambers Street that fits your description with outdoor space, but again I am not sure if this is the type of project you are looking at....
http://www.streeteasy.com/nyc/sale/90871-condo-200-chambers-street-tribeca-new-york
Sift through the sales you find on public records with StreetEasy, and see which of the sales was listed on streeteasy by entering the address in the search bar, if it was listed, see if the realtor listed any outdoor space, you may click on the listing agents name for example "Corcoran", and it will like you in directly to the agents listing page, Corcoran will leave a a sold listing on their website, others won't.
You may also try to download the condo declaration from ACRIS to see which units list outdoor space, I know it's a pain in the *** especially if you were lucky to get this assignment from an AMC who pays peanuts....
If the building you are talking about is 52 Thomas St.? then I did a unit is this building without outdoor space, are one of these units the subject?
http://www.streeteasy.com/nyc/sale/136633-condo-52-thomas-street-tribeca-manhattan
http://www.streeteasy.com/nyc/sale/47746-condo-52-thomas-street-tribeca-manhattan
Note: if you sign up with StreetEasy (it's free) it will give you info on all the units, otherwise it will only list the last three sales.