Can you guarantee successful Planning Approval ?
Making a Planning application is rarely a foregone conclusion, and the outcome can never be guaranteed, but adhering to Relevant Guidance and presenting a scheme with a supporting rationale in a 'Design Statement' will usually make it much easier for the Local Planning Authority (LPA) to approve.
Smaller 'Permitted Development' rights allow modest extensions without the need for a formal Planning application, but it is usually advisable to apply for a 'Certificate of Lawfulness' to confirm this.
Planning applications occur in a public forum, and submitted drawings are usually posted on-line for viewing and comment by neighbours. Whilst Planning law is clearly defined, there are degrees of interpretation of Relevant Guidance according to context that can appear to be more subjective. LPA's do strive to be consistent and accountable in their decisions, so the approval process is as much a political as a design one, taking into account similar projects in the area that may have been approved or refused.
A pre-application enquiry consultation with the Local Planning Authority (LPA) can sometimes generate an informal opinion as to the likelihood of success and early dialogue might reduce the risk of encountering obstacles later. LPA's charge for this pre-submission advice.
Ultimately only a formal application can determine the outcome.
Even with a successful Planning Approval, other obstacles can emerge that prevent development. 'No build' policies above public adopted sewer runs later discovered under the site are a particular constraint. Records are not always reliable and such restrictions are not necessarily identified explicitly by Statutory Consultees during the Planning Process. Build-over Sewer Agreements are technically feasible and often do get agreed and implemented on site, but may be prevented in principle by official water company policy (especially new build development).