The "return" part of the investment will be your own enjoyment of the space. Any increase in property value will likely be overall market movement, rather than that particular spend, so treat it as a bonus.
Trying to figure out "if I spend X, I'll get Y back when we sell..." is a complete guess.
Do it because your family want to enjoy it - it'll feel like a much better "investment" than something you can enter into Excel.
Edit: We've been quoted a similar amount to dig up the monstrous concrete wheelchair ramp, bring the garden up to house level (about 70cm) pave over our entire garden and replace the back fence with a brick wall to match the end of terrace boundary.
We'd do it at the same time as adding bi-folds/patio doors from the dining area, and brick up the existing back door.
If we use the same porcelain tiles we want for the kitchen/lounge/diner to extend out into the garden (inc. French drain), then we're looking at approx £30k all in.
We'd definitely get a return on the spend, as the existing garden is awful as-is, but it's difficult to quantify with any accuracy.
The house is a wheelchair accessible, 4 bed 2 bath end of terrace council box (Flat Roof, PRC construction), which we're buying for £118k, but valued at £205k (without any consideration for internal/external decoration/modifications). We'll need to hold onto it for at least 5 years, then sell for market value (£275k+ hopefully), which would give us a return of roughly 120k (and allow us to jump up a few rungs on the ladder, to a £450k+ property).
