Three things to get right before you start your home extension
Most home extension projects that run into trouble don’t go wrong on site. They go wrong before a single brick is laid — in the weeks and months beforehand, when key decisions were either rushed, deferred, or never properly made at all. A builder arrives and the drawings aren’t ready. A planning application takes longer than expected because the brief kept changing. A budget gets agreed without accounting for half the costs involved.
None of this is inevitable. The homeowners who have the smoothest, least stressful extension projects are rarely the ones with the biggest budgets or the simplest sites. They’re the ones who took the time at the beginning to get three things straight: their budget, their layout, and their planning requirements. Everything else follows from those.
1. Budget and costs — set it clearly and set it early
Budget is the conversation that most people find uncomfortable to have and most designers find essential to have early. It shapes every decision that follows — how large the extension can be, what quality of materials is realistic, how much design time is appropriate, whether certain structural approaches are viable. A project without a clear budget is a project without a frame of reference, and that tends to lead to scope creep, disappointment, or both.
The first thing to understand is that the build cost is only one part of the total investment. Design fees, planning application fees, building regulation fees, and structural engineer fees all sit alongside the build cost, as do party wall surveyor fees where applicable and any enabling works such as groundworks or drainage diversions. All of these need to be accounted for from the outset rather than discovered as the project progresses.
Professional fees for a residential extension project are typically a modest percentage of the overall build cost — four to nine percent— and a contingency of around ten percent of the build cost is sensible on top of that. Ground conditions, unforeseen structural issues, and changes of mind during the build are the three most common sources of cost overrun, and a realistic buffer protects against all of them.
The other budget conversation worth having early is about value. The best-designed projects in the Surrey market — those that add a bedroom, improve the ground floor layout, or meaningfully increase natural light — tend to add more in property value than comparable unimproved homes, though returns vary depending on the property, the quality of the work, and market conditions at the time. Understanding roughly where your project sits in that equation helps prioritise where the budget is best spent.
2. Layout and space — think about how you’ll actually live in it
The layout question is deceptively simple: how do you want to live, and does your current home support that? The extension is the opportunity to close the gap between those two things. But closing that gap requires understanding it first — which means thinking honestly about how your household actually uses the space it has, not how you imagine it does.
Which rooms feel too small? Which feel unused? Where does the family actually congregate, and is that where the design of the house intended them to congregate? Where does the morning routine break down? Where does the clutter accumulate? These questions sound basic, but the answers reveal the real brief — which is often quite different from the initial instinct of “we need a bigger kitchen” or “we need another bedroom.”
Layout decisions made at the design stage are cheap. Layout decisions unmade at the design stage and discovered on site are expensive. The connection between the extension and the existing house — how they flow together, where the thresholds are, how light moves through the combined space — needs to be resolved on paper before anyone picks up a tool. A poorly considered junction between old and new is one of the most common sources of disappointment in finished extension projects, and it’s entirely avoidable.
For families in the early stages of thinking about a project in Surrey or Hampshire, the most useful thing to do before any conversation with a designer is to spend a week paying attention to how your home works and where it lets you down. Not the things you’d change aesthetically — the things that genuinely affect how you function as a household. That information is the raw material from which a good brief is made.
3. Planning and regulations — understand the requirements before you commit
Planning permission and building regulations are two separate things, and understanding both — and which applies to your project — before you commit to a design direction saves significant time and money.
Planning permission is about what you can build: its size, appearance, and relationship to the street and neighbouring properties. Many single-storey rear extensions fall within permitted development rights, meaning no formal planning application is required. But permitted development has limits — on size, on height, on how close the extension can come to boundaries — and projects that exceed those limits need a householder planning application, which takes a minimum of eight weeks and may result in conditions or modifications being required.
The earlier you understand which route your project takes, the better. A design developed in the assumption that permitted development applies, and then discovered to require a full planning application, needs to go back to the drawing board in a way that costs time and sometimes money. Establishing the planning position at the outset — before the design is too far advanced — avoids that entirely.
Building regulations are separate and apply regardless of whether planning permission is needed. They cover how the extension is built: structure, insulation, drainage, ventilation, fire safety. Building regulation approval is obtained through the building regulation drawings stage, after planning is resolved, and is checked by a building control inspector during the build. Understanding that this stage exists — and budgeting time and fees for it — means it doesn’t come as a surprise halfway through the project.
For properties in conservation areas, on listed buildings, or in areas subject to Article 4 directions — which remove some or all permitted development rights — the planning requirements are more involved, and early advice is particularly important. We work regularly with homeowners across Frimley, Deepcut, Lightwater, and the wider Surrey area and can give a clear steer on the planning position for any given project from the first conversation.
Why getting these three things right changes everything
A home extension is one of the most significant investments most families make. The difference between a project that runs smoothly and one that doesn’t is rarely about what goes wrong on site — it’s about how well-prepared the project was before it got there. A clear budget prevents scope creep and protects against the surprises that derail timelines. A well-considered layout produces a home that genuinely works better rather than one that’s simply larger. And an early understanding of the planning position means the design can be developed with confidence rather than revised under pressure.
These three things aren’t complicated. They just require attention before the exciting parts begin — before the design conversations and the material choices and the builder quotes. The homeowners who give them that attention consistently have better projects.
FAQ
How much should I budget for a home extension in Surrey?
Build costs for a single-storey rear extension in Surrey typically start from around £2,000–£2,500 per square metre, though this varies with specification, ground conditions, and market conditions at the time of tender. Professional fees — design, planning, structural engineering, building control — typically add fifteen to twenty percent on top of the build cost. A contingency of ten percent of the build cost is sensible in addition to that. We provide a clear breakdown of the full expected cost at the initial consultation stage.
How do I know if my extension needs planning permission?
The starting point is whether the project falls within permitted development rights, which allow certain extensions to be built without a formal planning application provided they meet specific criteria. Whether your project qualifies depends on its size, height, position, and the designation of your property and area. We establish this at the outset of every project — it’s one of the first things we advise on, and it shapes the design approach from the beginning.
What happens if I start building without the right approvals?
Building without planning permission where it’s required, or without building regulations approval, creates problems that tend to surface when you come to sell. A buyer’s solicitor will request evidence of the relevant consents and sign-offs — and if they can’t be provided, the sale stalls. Retrospective approval is possible in some cases but is more complicated and more expensive than doing it correctly in the first place. The straightforward answer is to get the approvals in place before building starts.
Ready to start planning your project?
If you’re at the early thinking stage — or if you’ve been putting it off because it all feels a bit complicated — we’d be happy to have a straightforward conversation about what’s involved. We work with homeowners across Surrey, Hampshire, Berkshire, and London, and most projects start with nothing more than a chat about what you’re hoping to achieve.
Get in touch at jakeunderwood.com or call 07784 927778.
