If you know your foundation is sinking and you're ready to call a house underpinning service, it's important not to delay doing this, even if the damage that the subsidence is doing to your house is happening very gradually. Here are two reasons why.
1. The underpinning work will become more expensive and complex if you postpone it
The longer you wait to have the house underpinning done, the more complex and costly it will be for your contractor to do it. For example, if your foundation is still in the early stages of sinking and you get it underpinned now, your contractor may only need to fit one or two concrete piles to stabilise this section and then do a little work to eliminate the source of the subsidence (such as cutting the tree roots that drew moisture from the earth under the foundation and made the soil contract).
In contrast, if you ignore this issue because the damage it's doing is not, as of yet, that noticeable, the sinking will continue and as it does, more and more of the foundation will sag downwards into the ground. This will not only result in the contractor having to fit more piles but will also mean that they'll have to do more repairs to the foundation itself (such as replacing cracked sections of the foundation).
Additionally, you'll also find that the previously subtle issues throughout your home that were caused by this issue become more visible and that you need to address them to restore the aesthetics of your home (for example, you might need to repair the initially thin diagonal cracks on your walls that eventually widened and became unsightly).
2. The 'subtle' initial subsidence issues may make other renovations impossible to do properly
Even if the issues throughout your home that are being caused by your sagging foundation seem subtle, if you don't use an underpinning service, you may find that it becomes impossible to do renovations whose effects are long-lasting or satisfactory.
For example, if you want to update your windows or doors, you might find that the seemingly minor but continuous warping of the window and door frames that are the result of your unrepaired foundation might lead to the new doors and windows 'sticking' and the varnish on their frames being scraped off or scratched as a result of them not fitting correctly. As such, if you're hoping to make other improvements to your home soon, you should prioritise the house underpinning so that the results of the former will last.