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Planning an extension is exciting. It is easy to get swept up in Pinterest boards, bi-fold doors, and imagining that new open-plan life.
But between the dream and the reality lies a minefield of regulations.
At Plan My Extension, we see the same issues crop up repeatedly. Mistakes made at this early stage do not just cause headaches—they cause delays, force expensive redesigns, and can even leave you with an unlawful building.
Here are the most common pitfalls to watch out for, so you can get it right the first time.
- Assuming “Permitted Development” is a Free Pass
Many homeowners assume that because their project looks simple, it is automatically Permitted Development (PD).
The Trap: PD rules are incredibly strict. If your extension is just a few centimetres too high, or the eaves material does not match the existing house, the entire build can be deemed unlawful. We have seen projects halted because of minor technicalities that were overlooked in the excitement.
The Fix: Never guess. Get professional drawings done first to confirm the measurements and ensure you check every single condition in the technical guidance before a brick is laid.
- The “My Neighbour Did It” Fallacy
You see a loft conversion three doors down and assume you can do the same. Not necessarily.
The Trap: Planning rights can vary wildly even on the same street. If you live on a newer estate, or if your property was converted from a shop or office, your PD rights might have been removed (often called an Article 4 Direction). Just because Number 42 got away with it, does not mean you will.
The Fix: Always check your specific property’s planning history. Do not rely on precedence; rely on the paperwork.
- Designing Without a Budget Strategy
It is heartbreaking to see homeowners fall in love with a design, pay for approvals, and then realise the build cost is double their budget.
The Trap: Sketching a “dream home” without factoring in the current cost of labour and materials leads to rushed, stressful redesigns later on—or a project that stalls halfway through.
The Fix: Be realistic from day one. Design with a budget range in mind and keep a healthy contingency fund. A good set of plans should be buildable, not just beautiful.
- Starting a War with the Neighbours
You might have the legal right to build, but that does not mean your neighbours will not object.
The Trap: Ignoring how your extension affects others—blocking their light, overshadowing their garden, or building right up to the boundary line—is the fastest way to trigger a dispute. Disputes cost time, money, and emotional energy.
The Fix: Design with impact in mind. If you are going near the boundary, talk to your neighbours early. A cup of tea and a chat over the plans can prevent a formal objection later.
- Relying on “Back of a Napkin” Sketches
Some builders will say they can work off a basic sketch. This is risky.
The Trap: Planning Officers and Building Control inspectors need precise, technical information. Vague drawings leave room for interpretation, which leads to mistakes on site and arguments over who pays to fix them.
The Fix: Invest in proper, compliant drawings. They are your contract with the builder and your safety net with the council.
- Confusing Planning with Building Regs
“I don’t need planning permission, so I can just start, right?” Wrong.
The Trap: Planning deals with how the building looks and impacts the area. Building Regulations deal with whether the building will stand up, stay warm, and be safe in a fire. Even if you have PD rights, you always need Building Control approval.
The Fix: Ensure your project timeline includes Building Regulations drawings (structural calculations, insulation, drainage) alongside your planning checks.
- Skipping the Paperwork (The Lawful Development Certificate)
This is the one that bites people years later.
The Trap: You build under Permitted Development but never get official confirmation from the council. Five years later, you try to sell your house, and the buyer’s solicitor demands proof the extension is legal. You have none, and the sale collapses.
The Fix: Always apply for a Lawful Development Certificate. It is the “golden ticket” that proves your build was legal at the time. It is a small cost now to protect your property’s value forever.
How to Get It Right from Day One
You do not need to become a planning expert to build your dream extension—you just need the right partner.
At Plan My Extension, we help you navigate the maze.
- We check if your home qualifies for Permitted Development.
- We produce clear, technical drawings that builders love.
- We handle the council applications for you (including that all-important Lawful Development Certificate).
Ready to start planning properly? Stop guessing and start building. Get an instant fixed-fee quote online and let us get your project moving.
