Grant funders and commissioners all expect applicants to demonstrate the impact of what they do. There are at least three highly regarded sources of advice/methodologies on impact:
Surrey Impact Framework
Surrey Impact Framework - This is a free management tool, developed as part of the Transforming Local Infrastructure funded Supporting the Frontline project which has been designed specifically to help organisations in Surrey to take a fresh look as their operations. For more see: http://www.surreyca.org.uk/services/surrey-impact-framework/
Charities Evaluation Service
The CES Resource Guide: Evaluating outcomes and impact
A report Are you leading for impact? by ACEVO & Inspiring Impact - April 2013
The Young Foundation
The Young Foundation - They have done specific work for the youth sector on measuring impact. Their methodology is sumarised as a 4 Box Model that distinguishes between extrinsic and intrinsic outcomes and those affecting individuals and impacting on Society.
The emthodology is very clear, ideally suited to youth work and very accessible.
Visit this link to see the relevant documents: http://youngfoundation.org/publications/framework-of-outcomes-for-young-people/
New Philanthropy Capital
New Philanthropy Capital - They also have a high reputation for work on outcome measurement. Visit this link to see the relevant documents: http://www.thinknpc.org/publications/making-an-impact/
National Youth Agency
National Youth Agency - The Quality Mark referred to above is also relevant to outcome measurement.