What to look for when planning a house extension

A clear vision and purpose
Before you even speak to a builder, define why you’re extending.
• Do you need a larger kitchen?
• A home office?
• A better connection to the garden?
A clear brief helps your designer create a space that genuinely improves your home’s layout and value.

Early planning and legal checks
Extensions often require navigating planning permission, permitted development rules, and building regulations.
• Check whether your project falls under permitted development.
• Consider constraints such as conservation areas or party wall agreements.
Understanding legalities early prevents costly delays.

A realistic budget (with contingency)
Extensions typically cost £1,000–£3,000 per m² depending on quality and complexity. Make sure your budget includes:
• Professional fees
• Surveys
• Structural work
• VAT
• A 10–15% contingency for surprises

A qualified designer or architect
Skipping a designer is one of the most common mistakes homeowners make. A good designer will:
• Maximise natural light
• Improve room proportions
• Ensure the extension complements your existing home
• Help navigate planning and building control

A reputable, vetted builder
Look for builders who:
• Are members of recognised bodies (e.g., FMB)
• Provide detailed written quotes
• Have strong references and previous projects you can visit
• Offer clear timelines and communication plans

Consideration for natural light
Natural light is one of the biggest factors affecting how an extension feels. Rooflights, glazing, and thoughtful orientation can prevent the new space from feeling dark or boxed‑in - a common issue when extending terraced or semi‑detached homes.

Good project management
Whether you manage the project yourself or hire a professional, you need:
• Regular site meetings
• Clear communication
• A plan for decision‑making
• A record of changes and costs

What to avoid when building a house extension

Poor communication
Communication breakdowns is a major cause of delays and disputes. Avoid:
• Vague instructions
• Assumptions
• Unrecorded changes
Clear communication = fewer surprises.

Collecting too many quotes
It’s tempting to gather endless quotes, but this can cause confusion and delay. Aim for three detailed, like‑for‑like quotes from reputable builders.

Underestimating the disruption
Extensions are messy. Dust, noise, and temporary loss of space are inevitable. If possible, avoid living on‑site during major structural work - it slows the builders down and increases stress.

Sacrificing natural light for floor area
Bigger isn’t always better. A poorly lit extension can feel gloomy and disconnected. Prioritise:
• Glazing
• Rooflights 
• Internal sightlines
• Garden views

Using inappropriate materials
Choosing materials that don’t suit your existing home can cause planning issues and reduce resale appeal. Avoid mismatched brickwork, clashing rooflines, or cheap finishes that won’t stand the test of time.

Ignoring room ratios
A huge open‑plan space with a tiny utility or awkward leftover corners can feel unbalanced. Designers stress the importance of proportion and flow -  not just square footage.

Forgetting the hidden costs
Beyond the build itself, you may need:
• Upgraded electrics
• Drainage changes 
• Structural steel
• Landscaping
• New heating solutions
These can significantly increase the final cost if not planned for upfront.

A house extension is one of the best ways to enhance your home - but only when approached with clarity, planning, and the right team. By focusing on design quality, legal requirements, and communication, while avoiding common pitfalls like poor budgeting or sacrificing natural light, you’ll set your project up for success.