If you don’t feel up to the task of fitting French doors yourself, you can hire a tradesperson to do it for you. The cost of any work like this will depend in part on your location. You can expect to pay more for labor costs in London for example, than say, Derbyshire. However as a general rule, the average cost to install internal French doors (assuming there are no complications involved) should be around £100 - £150. You’ll pay extra if you want old doors removing, and extra again if you want them disposed of. Needless to say, these costs are for installing French doors in an existing opening. Costs will increase significantly if you require an opening to be created.