Extending your home is a major investment—financially and emotionally. While the results can be transformational, the process is rife with potential errors. In this comprehensive guide, KT5 Construction shares the 17 most common house extension mistakes homeowners make, and how you can avoid them to ensure a smooth and successful build.

1. Underestimating the Budget

Failing to budget realistically is a classic pitfall. Costs can quickly spiral due to material choices, custom features, or unplanned issues.

Avoid it:

• • Itemise every cost.
  • Include a 10–20% contingency.
  • Use KT5’s free online cost calculator for a fast, accurate estimate.

2. Engaging Professionals Too Late

Bringing in architects or structural engineers after you’ve made design decisions can limit your options and add to costs.

Avoid it:

• • Consult professionals early.
  • KT5 Construction works collaboratively from day one to optimise your vision.

3. Skipping Surveys (Like Drainage and Soil)

Ignoring exploratory surveys can lead to costly surprises with foundations and drainage.

Avoid it:

• • Conduct CCTV drainage surveys and trial pits.
  • KT5 offers early investigative work to avoid structural or planning setbacks.

4. Forgetting Party Wall Agreements

If your building is close to a neighbour’s boundary, a Party Wall Agreement is likely required.

Avoid it:

• • Notify neighbours early.
  • KT5 handles Party Wall compliance for you.

5. Unrealistic Timelines

Expecting a complex extension to be completed in a few weeks often leads to frustration and rushed decisions.

Avoid it:

• • Plan with time for design, permissions, build, and potential delays.
  • KT5 builds accurate schedules with clear milestones.

6. Rushing the Design Stage

Jumping into construction with half-baked plans often results in poor flow, layout issues, or regrets.

Avoid it:

• • Spend time on design.
  • KT5 offers expert design services tailored to your lifestyle.

7. Not Considering Natural Light

Extensions can block existing windows and reduce internal light.

Avoid it:

• • Add roof lights or glazed panels.
  • KT5 balances structure and light for healthy, bright homes.

8. Poor Storage Planning

Not planning where furniture, tools, or kitchen units go during work causes chaos.

Avoid it:

• • Use temporary storage or plan logistics.
  • KT5 guides you through site setup and storage options.

9. Living on Site Without Preparation

It’s doable, but dusty, noisy, and disruptive without a plan.

Avoid it:

• • Section off living areas.
  • KT5 helps you maintain safe, liveable conditions during the build.

10. Choosing the Cheapest Quote

Cheapest doesn’t mean best. Poor work or delays can cost more later.

Avoid it:

• • Vet contractors.
  • KT5 provides transparent, fair quotes with guaranteed quality.

11. Ignoring Weather and Seasonality

Building through winter can increase delays and costs.

Avoid it:

• • Schedule disruptive works in better weather.
  • KT5 factors seasonal impacts into the timeline.

12. Not Having a Schedule of Works

Without a plan, projects easily stall or go over budget.

Avoid it:

• • Use a clear, phase-based timeline.
  • KT5 keeps you informed with detailed schedules

13. Lack of Insurance and Site Safety

Building work can void your home insurance without proper cover.

Avoid it:

• • Get extension-specific insurance.
  • KT5 is fully insured and guides you on coverage needs.

14. Making Design Changes Mid-Build

Changing layouts, windows or materials after starting adds delays and costs.

Avoid it:

• • Finalise all decisions pre-construction.
  • KT5 conducts thorough pre-start reviews.

15. Not Snagging at Each Stage

Minor defects build up if you skip regular checks.

Avoid it:

• • Inspect each phase with your builder.
  • KT5 conducts regular snagging to ensure flawless delivery.

16. Neglecting Sustainability

Missing out on energy-efficient features can raise bills long-term.

Avoid it:

• • Add insulation, energy-efficient glazing or solar.
  • KT5 integrates eco-options without overspending.

17. Forgetting to Plan for Amenities

Tradespeople need access to water, electricity and sometimes toilets.

Avoid it:

• • Arrange site facilities.
  • KT5 provides portable site toilets and utility setups as standard.

Why Most Homeowners Choose KT5 Construction

KT5 Construction is a trusted name throughout Surrey and London, renowned for our commitment to excellence, transparency, and client satisfaction. Homeowners choose us for:

• • Over 25 years of proven experience in house extensions
  • Comprehensive project management from concept to completion
  • Transparent, itemised quotes with no hidden costs
  • Superior craftsmanship and tidy, respectful teams

When you work with KT5, you are choosing a team that treats your home with the utmost care and professionalism.

Final Thoughts on House Extension Mistakes

Avoiding these 17 house extension mistakes will help ensure your project is a rewarding investment. KT5 Construction has been building smart, stylish, and stress-free home extensions for over 25 years across Surrey and London.

Ready to Build ? 

Contact KT5 Construction today for a free home consultation. Our team of trusted experts will guide you through every step—from design to final inspection—while helping you save time, money and stress.

Call us now or use our free extension cost calculator to get your instant quote in just 60 seconds!