There are plenty of ‘tips’ but, if you’ll deal with several companies, seek professionals to drive the process and optimise (also financially) your renovation project.
If you’re planning to improve the energy performance of your house - exterior insulation, or change the heating system, or better windows, or solar panels - check the local authorities. Sometimes there are subsidies/grants to support a part of the renovation costs. Also very important: don’t forget to ask for an asbestos screening before any intervention if it’s an old house. And if you’re planning to start a significantly amount of renovations yourself, check if a small insurance can be useful.
And don’t forget to check the building potential of the plot… maybe you can make it bigger, and it will increase size and value.