Planning a House Extension: A Complete Guide
- Jan 30, 2019
- 5 min read
Planning a house extension is one of the most effective ways to add space, value, and functionality to your home, without the cost and disruption of moving. But it's not a small undertaking. Done properly, a house extension can transform how you live. Done poorly, it can cost far more than expected and cause months of stress.
At Astrum Construction, we've helped homeowners across Northamptonshire, Milton Keynes, Buckinghamshire, and Bedfordshire plan and build extensions of all shapes and sizes — from modest kitchen extensions to full multi-storey additions. This guide walks you through every stage of the process.
1. Define what you actually need
Before speaking to an architect or getting quotes, spend time understanding what problem you're trying to solve. Are you short on kitchen space? Do you need an extra bedroom? Is your living room too small for how your family uses it?
The clearer your brief, the smoother the design process will be and the less likely you are to make expensive changes mid-build. Write down your must-haves and your nice-to-haves before any professional conversations begin.
2. Set a realistic budget — including contingency
House extension costs in the UK typically range from £1,500 to £2,500 per square metre, depending on the complexity of the build, the materials used, and your location. A single-storey rear extension might cost £30,000–£60,000, while a double-storey extension could reach £80,000–£150,000 or more.
Always build in a contingency of at least 10–15%. Even well-planned projects encounter unexpected issues — especially in older properties where hidden structural problems can emerge once work begins.
Your budget should account for:
Architect and structural engineer fees
Planning application costs (if required)
Building regulations approval
Construction costs (labour and materials)
Internal fit-out — flooring, plastering, decoration
Any utility upgrades (plumbing, electrics)
3. Understand planning permission vs permitted development
Not all extensions require planning permission. In England, many single-storey rear extensions fall under permitted development rights — meaning you can build without a full planning application, as long as your project stays within certain size limits.
However, permitted development rules have conditions and exceptions — particularly for listed buildings, properties in conservation areas, and flats. It's always worth checking with your local authority or speaking to a planning consultant before proceeding.
If your extension does require planning permission, allow 8–10 weeks for the application to be decided. During this time, your architect can be refining the technical drawings so you're ready to start promptly once approval comes through.
4. Hire the right professionals
A successful extension depends on the quality of the team around you. At a minimum, you'll need:
An architect or architectural designer to produce drawings and plans
A structural engineer to sign off on any load-bearing changes
A reputable builder or main contractor to manage the construction
When choosing a builder, look for evidence of similar completed projects, proper accreditation (such as Trustmark or Federation of Master Builders), and clear, detailed quotes. Avoid anyone who asks for large cash deposits upfront or can't provide references.
Astrum Construction manages the full process from initial design consultations through to build completion, removing much of the complexity for homeowners. Get in touch to discuss your project.
5. Think carefully about design and integration
One of the most common mistakes homeowners make when planning a house extension is treating the new space as completely separate from the existing building. The most successful extensions feel like they were always part of the house.
Work with your architect to consider:
How the extension connects to existing rooms — flow matters enormously
Natural light — bi-fold doors, roof lanterns, and skylights can transform a space
Materials — matching or complementing your existing brickwork, stonework, or render
Future flexibility — could the space serve multiple purposes over time?
6. Plan for building regulations approval
Even if your extension doesn't need planning permission, it will almost certainly need to comply with building regulations. These cover structural integrity, fire safety, energy efficiency, ventilation, and drainage — among other things.
Your builder should submit a building notice or full plans application before work starts. At key stages during the build, a building control officer will inspect the work. At the end, you'll receive a completion certificate — an important document if you ever come to sell your home.
7. Prepare for life during the build
An extension is disruptive. There's no way around it. Dust, noise, and restricted access to parts of your home are part of the process. The key is planning ahead:
Discuss site access, working hours, and welfare facilities with your builder before work starts
If your kitchen is being extended, think about how you'll cook during the build
Protect furniture and belongings in adjacent rooms from dust
Keep communication with your builder regular — weekly check-ins prevent small issues becoming big ones
8. A real example — from 2 bedrooms to 4
One of our recent projects in Milton Keynes illustrates what careful planning can achieve. We transformed a modest two-bedroom semi-detached house into a spacious four-bedroom family home through a substantial rear and side extension.
The clients came to us with a clear brief: they needed more bedrooms, a larger kitchen-diner, and better flow between the living spaces. Working closely with them from concept through to completion, we designed an extension that felt entirely native to the existing property — matching the brickwork, maintaining the roofline, and introducing a large glazed rear elevation to flood the new kitchen with natural light.
The transformation was significant, and the moment the clients walked through the finished space — seeing the design come to life — is one of the most rewarding parts of what we do.
9. Energy efficiency and sustainability
Building an extension gives you the opportunity to incorporate modern energy-efficient features that would be costly to retrofit later. Consider:
High-performance insulation in walls, floor, and roof
Triple-glazed or high-spec double-glazed windows and doors
Underfloor heating — particularly effective in open-plan extensions
Solar-ready roof structures if panels are on your long-term radar
These choices add modest upfront cost but reduce running costs over time — and increasingly matter to buyers if you come to sell.
Ready to start planning your house extension?
Planning a house extension is a significant undertaking, but with the right team and the right preparation, the results are transformative. Astrum Construction has over 20 years of experience delivering extensions across Northamptonshire, Milton Keynes, Buckinghamshire, and Bedfordshire.
We handle everything from initial design through to final handover — so you can focus on looking forward to the finished result. Contact us today for a free consultation.
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