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

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

1. Network management costs: The network operator may charge for modifying the connection and installing a new meter. These costs differ per network operator and can range from several hundreds to thousands of euros.
   
   

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

3. Any additional costs: Depending on the specific situation, there may be other costs, 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 assist in assessing the necessary adjustments and providing a quotation for the electrical installation work.