My question is: is the 10% of yearly revenue a reasonable amount to target in savings with an eye towards improving the properties?
That's impossible to answer, because it depends on the location.
A landlord's revenue is rent. The problem of using rent as a basis for saving towards renovations is that:
- Some locations have a high rent to house price ratio, other locations have a low rent to house price ratio. Surely you don't want the future of your property depending on the expensiveness of the area (unless the area is so cheap it doesn't make sense to renovate)
- In some locations, the land is by far the largest part of house prices. In other locations, the cost of land is (nearly or exactly) zero and the cost of a house only consists of the cost to build it. In fact, there can be an order of magnitude of difference in house prices, mainly due to this. Just compare some rural area to New York, San Francisco or similar. Houses do need renovations, but land doesn't. While there can be slight differences in cost to construct (which affects the price of renovations too), the differences are not as large as differences in house prices because land doesn't need renovations.
My usual rule of thumb is: you should save annually 2% of the cost to construct a new house every year to finance renovations. This means that in an accounting software, you would depreciate the house in 50 years. (Contrary to a popular myth, houses do not appreciate -- they depreciate).
This means that a house on leased land that costs 2500 euros per square meter, you should save 50 euros per square meter per year. Note that as cost to construct increases due to inflation, so should your savings.
If you buy a new house that's not on leased land in an expensive area, you might perhaps expect to pay 6000 euros per square meter. That consists of 3500 euros per square meter (land) and 2500 euros per square meter (house). The amount you should save is the same, because land doesn't need renovations.
For example, I pay 139 euros per year per square meter for my kind-of-rental house. Saving 10% of that would be 13.9 euros per year per square meter, not at all enough for renovations. So that's probably the reason why I live in this kind-of-rental property: it's just so cheap.