The One-Page Proposal Template is an outline for your proposal. This outline 
captures the key concepts of your proposal idea in an easy to communicate way. The goal of the one-page outline is to get other people interested. The outline concentrates on the basic idea and objectives of the proposal, together with key results and the impact. Describe the expertise needed and the users who benefit from your idea. Additionally the one-page outline makes it clear to all partners that you are the “owner” of the idea. Reciprocally list all other contributors once they and you have decided to work together and they have contributed. I used a short outline already years ago for other proposals than Horizon 2020, and I re-discovered these one-page outlines again through the work of Joe Gorman and Sean McCarthy in the context of EU proposals.
As an example, the one-page outline could have the following form:
Title:
Try to find a descriptive title and an acronym.
Contact:
Name, email, organisation and telephone.
Proposal idea:
Description of the idea in no more than 3-5 sentences.
Objectives:
What do you want to achieve with your idea?
European Dimension:
Why should this be an EU/Horizon 2020 project?
Results:
What concrete results will come out of your project?
Users:
Who can and will use the results of your project?
Impact:
What is possible once your project achieves its objectives and delivers its results?
Tentative Horizon 2020 work programme, call, and funding scheme:
This is needed for partner organisations to evaluate the financial and organisational feasibility.
Partners and roles:
Which functions and roles need to be filled?
Budget and duration estimate:
Budget in the order of thousands or millions of Euro?
Project duration in months or years?
The one-page outline is for sharing with other people. The main benefit is that you can use it as a basis for further discussion on your project idea, be it technical or in terms of partners needed.
It is recommended to use a system for yourself to keep track with whom you discussed your one-page outline and who you see as a potential collaborator. Once you start working with others on the one-page outline, it will expand as you add more and more information. This expanded outline then can serve as a (early stage) blueprint for a formal Horizon 2020 proposal.
You can download an empty template here as: horizonbook-one-page-template.pdf or horizonbook-one-page-template.docx. You can find more information in the book.