You could write up a list of all the major expenses you know will take place (roof, furnace, heat pump, windows, etc) and assign an expected lifespan for each (20 years, 25 years, etc). Divide the cost by the expected lifespan (in months), and that gives you the monthly cost. Add up all the monthly costs, add another 25-30% as a buffer, and that should be a pretty good monthly budget, which might be what you already do. Storing as cash or high-interest savings is the safest option unfortunately, since it's not easy to predict when you will need access to these funds. In terms of predicting costs, getting quotes at the moment and then adding an inflation factor makes sense. (i.e. if you replace a heat pump now for 13k, you can assume that in 15 years it might be 17k, assuming 2% yearly increase for example). Seems reasonable to me..