10 Common Mistakes London Homeowners Make When Building an Extension

Every year, thousands of London homeowners build extensions. Most go broadly well. Some go badly wrong. The problems are rarely unique — they cluster around the same preventable mistakes. Here are the ten most common and how to avoid each one.

1. Starting Without Confirming Planning Status

The mistake: Assuming the extension is Permitted Development because a friend's similar extension did not need planning permission. Starting work, then receiving an enforcement notice.

The fix: Obtain a Lawful Development Certificate before work starts. This costs approximately £206 and takes 8 weeks. It confirms in writing that your specific proposal is lawful. Do not rely on verbal assurances from a contractor or neighbour.

2. Underestimating the Total Budget

The mistake: Getting a quotation for the building work and treating that as the project cost. Then receiving the structural engineer's bill, the architect's bill, the planning fee, the Building Control fee, and the cost of the kitchen or bathroom that goes inside the extension.

The fix: Build a full project budget from the outset, including:

• Contractor cost (the building work)
• Structural engineer (£350–£900)
• Architectural designer (£1,500–£5,000+ for a standard extension)
• Planning fee (£258)
• Building Control fee (£300–£600)
• Party wall surveyors (£700–£1,500 if required)
• Materials you supply (kitchen, tiles, flooring, fittings)
• Contingency (minimum 10%, ideally 15%)

3. Choosing on Price Alone

The mistake: Getting three quotes and choosing the cheapest without investigating why it is cheaper. The cheapest quote is often cheap because something is missing, the contractor is underqualified, or the price will be made up in extras later.

The fix: Compare quotes by scope, not just by total. Ask what each quote includes and excludes. Check insurance, trade memberships, and references for all contractors. Choose on value and confidence, not the lowest number.

4. Not Allowing for Party Wall Obligations

The mistake: Starting structural works adjacent to the party wall without serving notice. Discovering later that the neighbour has grounds for an injunction, or that the work has caused structural movement that triggers a claim.

The fix: Identify whether your project triggers the Party Wall etc. Act 1996 before design is finalised. Serve notices at the right time (at least 2 months before notifiable works). Budget for party wall surveyor fees if the neighbour appoints their own.

5. Not Investigating Drainage

The mistake: Building a foundation over a public sewer without a Build Over Agreement from Thames Water. Discovering the issue when Building Control or Thames Water flags it — sometimes after the foundation is already poured.

The fix: Commission a drainage search and CCTV survey before design is finalised. If a public sewer is within 3 metres of the proposed foundation, initiate the Build Over Agreement process with Thames Water at the pre-application stage.

6. Ordering Kitchen/Bifolds Too Late

The mistake: Assuming kitchen units and bifold doors can be ordered once the building work is underway and will arrive in time. Discovering that bifolds have a 6–10 week lead time and kitchens have a 4–8 week lead time — both of which fall in the middle of the critical path.

The fix: Order bifold doors and kitchens at the latest when the planning permission is confirmed, ideally earlier if the design is fixed. Work backwards from your programme to establish the latest order date for each long-lead item.

7. Skipping the Structural Engineer

The mistake: Asking the contractor to "sort the beam" without a structural engineer's specification. The contractor installs a beam that is either undersized (structural risk) or massively oversized (wasted cost). Building Control refuses to approve the works without calculations.

The fix: Appoint a structural engineer before design is finalised. Their calculations are required by Building Control and protect both the project and the homeowner. The cost (typically £350–£900 for a residential extension) is not optional.

8. Moving In Too Early

The mistake: Claiming the space before it is genuinely complete — moving furniture into a room before the plaster is dry, using a new kitchen before all the connections are commissioned and certified.

The fix: Trust your contractor's programme. Plaster in a new extension needs 3–5 weeks to dry properly. Electrics and gas must be signed off before use. Moving in too early causes damage to newly finished surfaces, voids warranties, and sometimes creates safety risks.

9. Ignoring the Making-Good Scope

The mistake: Budgeting for the extension but not for the work needed to integrate it with the existing house — redecorating the rooms adjacent to the works, replacing flooring through the house, making good the kitchen that has been disrupted, replastering the area around the new steel beam.

The fix: Walk through the whole house with your contractor before finalising the scope and budget. Identify every area that will be disturbed by the works. Make sure the making-good scope is in the quotation rather than added as an extra later.

10. Not Getting a Completion Certificate

The mistake: Accepting the completed extension without following up on the Building Control Completion Certificate. Discovering years later, when selling the property, that the work was never signed off — and being asked to either regularise it or indemnify the buyer.

The fix: Make the Completion Certificate a contractual deliverable. Do not make final payment until the certificate has been received. File it with your property documents alongside the planning permission.

RCB Design & Build

RCB manages all of these risk areas as standard — planning confirmation, drainage investigation, structural coordination, programme management, party wall compliance, and Completion Certificate. Contact us to discuss your extension and how we structure projects to avoid the mistakes above.