OP - FWIW I think you should go for it if you can afford the 26k, you plan to be in the home for a while (which you do), and you do not really care about seeing a ROI (which it seems you do not). There is more to life than ROI, and you and your family will definitely get a lot of enjoyment out of the new space. Think of it this way - 26k gets you a fairly decent car these days. In 5 years the value of that car will be almost zip. A well cared for outdoor space can last 30 or more years, and will retain or even build value the whole time.
My wife and I are in the midst of major renovations at our place. New basement (35k), fully painted interior (6k), new master bath/laundry (40k), new mudroom (old laundry, 6k), face remodeled powder room (4k), reinsulated garage with new entry and vehicle doors (8k), new driveway (6.5k), new deck (3k - DIY) and full exterior paintjob (6k). Total 120k. I think I might see a ROI of 50-70% of the reno cost. Not because what we are doing is super nice, but because the house was a bit of dump when we bought it and was at the low end of the market in our neighborhood (we bought at 415k, houses in our hood go average 475-550k, neighbor's house just sold for 590k).
Ultimately I do not care about the ROI because the renovations we have already done have been a huge improvement, both to the home itself and my attitude toward it. When we started I felt like I was putting lipstick on a pig. Now I am starting to LOVE where I live. And while it has hurt to spend that much money (because I am a cheap bastard most of the time), it doesn't hurt TOO much because we can afford the project, and we don't plan to sell for a long time (20-30 years).