Yes, needs to be done under the car. Very easy. Disconnect the main supply under the bonnet in top right corner. After that remove all plastic at the bottom - hardest bit. You got then 2-3 bolts on starter and cables at the back.

I think starter must have 11 tooth. Try it out before you fit it in. it has to spin easy. You will see the difference to the one you remove. My old one did not want to spin on small battery at all! it felt like it was in short!

Good luck