-We intend to build an extension on the roof – possibly a floor and a half on a 20×45 structure, 3 stry house.
-We would set the extension a bit back from the front, so it is not so visible from the street.
– We would like lots of glass and light. V. modern in feel
– We would consider putting only rooms in the extension – without plumbing. However I would like to have an idea of what it would cost to have a small bathroom (toilet sink and shower), directly above 3rd floor bathroom. Would this be significantly more expensive ?
– Any idea of (1) who/architect has done this and (2) how much such an enterprise might cost ?
Any recommendations would be helpful

(My husband thinks that kits may be available, though I am doubtful)….


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  1. Is yours a frame house, or brick? I’m asking because I went through this with my house–even though zoning allowed an extra floor, it turned out that we couldn’t build any higher–even a 1/2 sized floor–on top of a 3 story frame building, per building code. Get an architect to give you an opinon on that, and if they tell you otherwise, please post their number, because I’d love to find a way around this myself!

  2. I think for what you are describing, “lots of glass, v. modern feel”, the price of 300k is a reasonable estimate. The cost will really be a function of the design as well as the existing structure and what kind of condition it is in. I am an architect and have done a number of modern extensions to townhouses and would be happy to discuss this further with you. Feel free to give me a call. 646.765.7228 – Mark

  3. figure the standard cost of $200 /sf…that is 1350 sf x 200= 270000..add into that the cost of underpinning the foundation to support the additional load and you get $250,000 on a good day…btw the kits that your husband is talking about run about $28,000 plus installation, and do not meet nyc code for habitable space