Once you have evaluated the significance of what you have (click here for more on Statements of Significance), the next step is to think about what you need and why.
Rather than just writing 'we want a meeting room in an extension on the north side', you should take a step back and write a list of all the things you'd like to be able to do. For example:
'We need a separate meeting room to hold PCC meetings in the warm, hold Sunday School without keeping our voices down, and hold the mother and toddler group in the church rather than the local café.'
When you have your list, you can start to look at possible ways of meeting those needs bearing in the mind what was highlighted as significant. Guidance on preparing a Statement of Needs is to be found here.
At this stage it's best to make an informal approach to the DAC to discuss your list of things you want to achieve in the building. They will likely want to have a site visit and will be able to identify any areas of difficulty or conflict which might arise. They will also be able to help with ideas of how to meet your needs if you don't have a particular solution in mind. Click here to get the contact details for your DAC.
If the DAC are broadly happy with the principle you can consider approaching an architect to draw up some initial plans. Your Statement of Needs can form part of the brief for your architect. You don't have to use the professional adviser who carries out your Quinquennial Inspections but they may be the best person for the job.
Visit other churches for inspiration of similar projects and to see evidence of professional's work.