Generally, the costs for upgrading from a single-phase to a three-phase connection can be significant.

Here are some potential cost items you may encounter when upgrading to a three-phase connection:

1. Network management costs: The network operator may charge costs for changing the connection and installing a new meter. These costs vary by network operator and can range from several hundred to thousands of euros.
   
   

2. Electrical installation costs: Upgrading to a three-phase connection may require modifications to the electrical installation in your home. This may include replacing or modifying the meter cabinet and installing new wiring. The costs for this can vary depending on the complexity of the work and the scope of the installation.
   
   

3. Possible additional costs: Depending on the specific situation, there may be other costs involved, such as permit fees or costs for hiring an electrician or installer.

To get an accurate estimate of the costs for upgrading to a three-phase connection, it is advisable to contact the network operator in your region. They can inform you about the specific costs and requirements for your situation. A certified electrician can also help assess the necessary modifications and provide a quote for the electrical installation work.