Work on the £150 million project will start in 2014 with the first of 15 3.3MW turbines being installed in the first half of 2015. The wind plant is due to be commissioned by the end of 2015 or early 2016.
The UK government’s energy secretary approved the project in February.
The extension is next to Vattenfall’s 30 turbine 90MW Kentish Flats site, which uses Vestas’s 3MW V90.
Vattenfall’s project manager Matthew Green said: "Vattenfall’s decision to invest around £150 million in Kentish Flats Extension is the final hurdle that needs to be cleared before construction can start in 2015.
"It’s good to get that and I’m looking forward to leading the construction team over the next two and a half years. We will now complete a period of intense detailed engineering and planning for the construction phase in 2015."
Vattenfall worked with Manston Airport in north-east Kent, to address the turbines' effect on the airport's radar system. In July C Speed won a contract to install Lightwave system to mitigate the potential impact on operations from the expansion.