I would really caution anybody who gives any weight at all to cutesy "value calculators" that tell you what kind of return you are going to be seeing from remodeling, or addition work. There are many reasons that these things tend to be wildly inaccurate and optimistic. Unless somebody is going to be buying your house for cash, and has no interest in actual value, the local market decides value, and appraisers determine exactly what those numbers are. For example, a basic two car garage addition in my area will run in the high twenty thousands range. The appraisal will be for $14-16K, since locally garages are values at $7-8K per bay, a tiny fraction of the number stated in the value calculator. There is also the concept of being functionally adequate. You own an old wreck of a place with a really sad kitchen, and decide that you will drop $20K into the kitchen, since it's a "good investment" well, good luck with that. The barely usable kitchen you had, without enough outlets on the counter, no dishwasher, and a fridge on the enclosed back porch was not considered to be functionally adequate, and would negatively impact the appraised value of your place. The new one you just dropped $20K on, did not raise the value by that amount, it addressed defects that existed, and made the place functionally adequate. It's no different that walking to the back of a house for sale and seeing a ledger nailed to the wall, where the deck should be, and a patio door above it leading to a six foot drop. The lack of a deck negatively impacts the value. Spending a few grand to install one, doesn't greatly increase the value.
In the OP's case, build a deck because it adds value to your life. Chances are it will increase the value of the place by at least what you spent on material, and that's a nice bonus, but when it comes to improvements and value, it's a tricky and often disappointing to see how little ROI there really is, in the end.
