Treat advertised “potential” as something to investigate, not as permission or a completed design assessment. [1][2]
Establish what exists and what was approved
Start with the council’s planning register. Obtain the actual decision notices and drawings rather than relying on an application number. Ask your designer to compare the house being sold with those records and identify any additions that need a separate explanation. GOV.UK provides a postcode route to the relevant council’s register. [3]
Separate two questions: is the existing development authorised, and what further development could be permitted? Permitted development rights have conditions and limitations, and can be restricted by a planning condition or an Article 4 direction. An assumption that every house has the same extension rights is unsafe. [4]
A useful brief for your designer is: “Identify a workable extension option, the likely permission route and the constraints that could stop it.” Include the rooms you need and what you would accept if the preferred layout is unavailable. Pre-application discussions can identify policy and information requirements, but council advice does not guarantee the eventual decision. [2]
Check what you own and may alter
Ask your conveyancer to review the title register, referenced deeds and any lease. The register can identify restrictive covenants and easements; the title plan normally shows general boundaries, not a guaranteed construction setting-out line. Do not use a printed title plan alone to decide where a new wall may go. [5]
For a leasehold property, put the intended extension into the legal brief from the outset. Buying a flat and having enough garden space do not, by themselves, resolve the rights and permissions in the lease. The government’s buying guide recommends understanding tenure and asking about alterations and restrictions before purchase. [1]
Test whether the project is physically realistic
Discuss the appropriate survey with a building surveyor. RICS identifies its Level 3 survey as suitable for altered, unusual or older buildings and homes where major work is planned. Agree any additional investigations expressly; a general survey should not be mistaken for an extension design or full investigation of concealed structure. [6]
Ask for a separate drainage check where the proposed footprint may cross a sewer. Thames Water’s developer guidance explains that building near its public pipes needs an agreement to protect the network and preserve access. These constraints should be assessed before settling the extension layout. [7]
Turn the checks into a buying decision
Use a short feasibility schedule with three columns: evidence obtained, unresolved issue and who will resolve it. List legal restrictions, planning uncertainty, structural investigations and drainage separately. This is a practical way of coordinating the conveyancer, surveyor and designer, whose investigations answer different questions. [1][6][2]
Do not make the purchase budget depend on an unverified extension. Ask advisers to explain which risks can be resolved before exchange and which remain your decision. A mortgage valuation is not a substitute for a condition survey. [1]