The difference is the question being asked
A proposed certificate asks whether the described work or use would be lawful if begun at the application date. It is determined against the relevant law and evidence, not through a discretionary judgement that the scheme deserves permission. [4][1]
A planning application asks the council to grant permission for development which is not otherwise authorised. An application to alter or enlarge one house may use the householder route; an application affecting a flat falls outside that simplified category. [2][5]
Choosing between these routes is not simply choosing between a cheaper and a more expensive form. The application must ask a question which fits the actual project.
A practical route-selection sequence
- Identify the whole project. Include the extension, roof or external alterations and any proposed change of use. Work out whether the operations constitute development and whether an existing permission already covers them. [2]
- Test any permitted development claim. Check the property, constraints, permission conditions and the complete applicable class. Use the original-house baseline and existing additions where the rules require them. [3][6]
- Identify procedural conditions. If prior approval is required, establish which class-specific application and outcome are needed before starting. [7][2]
- Choose the document which addresses the remaining question. A certificate can establish lawfulness; a planning application seeks a permission which is not otherwise available. [1]
For example, a qualifying extension which meets every applicable permitted development requirement may be suitable for a proposed certificate. A proposal beyond an applicable limit may need redesign or a planning application. That limit cannot be waived through a certificate merely because the neighbour supports the design. [1][3]
What if the problem is uncertain evidence?
A certificate can be refused because the applicant has not established the necessary facts. Where the missing issue is the original building or the meaning of an existing approval, better evidence may be the appropriate next step. Refusal does not always mean that a planning application was the only possible route. [1]
Read the reasons and distinguish a failure of evidence from a clear failure to comply with a legal condition. That distinction helps determine whether to revise the evidence, change the project, consider an appeal or seek permission. [1][8]
What if the work already exists?
An existing-development certificate asks whether the existing operations or use are lawful. A retrospective planning application seeks permission for unauthorised development. The fact that the work is finished does not make those questions identical, or remove the need for evidence supporting the chosen route. [1][9]
Keep the selected route aligned with the build
Whichever route is used, the description and drawings must correspond to the actual project. A later design change may need reassessment rather than being assumed to fall within the earlier certificate, permission or prior-approval decision. [1][10]
Finally, none of these planning documents replaces the building regulations process or resolves private rights. A correct planning route is one part of preparing the project, not a universal construction approval. [11][1]