Peer-to-peer (P2P) trading allows prosumers and consumers of electricity to exchange energy with each other outside the traditional centralised system. In this paper, we consider an eco-neighborhood in which individuals can invest in a renewable energy source (RES) and sell their excess power to the energy system at a given price, the feed-in-tariff (FiT) or to their neighbours on a P2P platform. We show that, in the absence of investment externalities, an appropriate FiT is sufficient to induce the first best investment and trade level and P2P has no value added. On the contrary, when negative externalities exist, P2P trading is necessary to restore efficiency.