Starting Your Extension Confidently: Tips from an Architectural Designer

Adding an extension to your house isn’t just adding more square footage, it’s about creating spaces that inspire, enhance your life every day, and unlock your property’s full potential. But what’s the best way to get from imagination to your dream home?

Architectural designer and chartered surveyor Mark Baldwin, founder of the London-based architectural practice plansdesigned, has decades of experience evaluating and designing properties – he knows the ins and outs of the extension process and its realities. Drawing on his thousands of completed projects and technical expertise, Mark shared his insight on everything from budgeting to how best to maximise natural light, so you can go into your extension informed and confident. Read on for expert tips and advice. 

1. Get crystal clear on costs

Something Mark would love homeowners to understand from the start is how much their extension plans are really going to cost. While online platforms and interiors magazines are amazing for offering inspiration, they rarely give an idea of total price or overall budget. So until you get an architectural designer or architect involved, it’s hard to know what’s achievable (which is exactly why The Page’s projects cite labor and material costs). Over the years, Mark’s been called in time and time again to “pick up the pieces” when homeowners have got started without a realistic budget for structural plans (or knowledge of what’s actually feasible in their home), and had to backtrack. 

Employing an architectural designer from the off can actually save you money. They’re well-versed on different structures, foundations, materials and details you can use in your extension – which can have a big impact on the total price. Not to mention the value that having clear architectural drawings to work off can add: these are the blueprints from which contractors can work off, making sure that your plans are brought to life just as you imagined. 

So financial planning is your first critical step to an extension. Homeowners in London can expect to invest around £1,500 to £3,000 per square metre for single-storey extensions, with two-storey extensions ranging from £2,000 to £3,500 per square metre.

Then you need to factor in the fittings, fixtures and finishes (which can amount to the same as the building’s shell) plus architect and structural engineers drawings, and a healthy contingency fund (about 20% of your project cost) to cover unexpected expenses or unforeseen building requirements. Labour costs vary hugely, but you should expect this to account for about two thirds of your final cost. Mark’s found that working on a fixed cost per project basis (as opposed to a percentage or day rate) is the most straightforward way of keeping to budgets and ensuring everyone is on the same page regarding requirements.

2. Think about what you want from your extension

House extensions and conversions come in all shapes and sizes, each with their own opportunities and considerations. The most popular Mark finds is a loft conversion: what’s not to like about turning a dusty unused storage space into beautiful new bedrooms and bathrooms? Garage conversions are also sought-after (particularly if you don’t have a car) as they can turn dead space into a home office or gym with relative ease. If you’re extending the front of your home, Mark advises that a well-designed porch can enhance your property and create a focal point for your front elevation – just make sure the entrance is “obvious and attractive”.

A rear extension can transform your living space, creating seamless indoor-outdoor connections and flooding your home with natural light. Rooflights (the same as skylights) and sunpipes (a kind of light tube) are Mark’s preferred way to make sure clients’ extensions feel bright and airy in addition to doors and windows. “Sunpipes are great for hallways or staircases, which are often quite dark,” he explains, “They reduce the reliance on artificial light and can really open up a space. It’s much nicer for people to come downstairs and have sunlight streaming through than to have to turn on a light switch.”

Side extensions offer clever solutions for properties with limited space, effectively utilising every square meter of your potential living area. Creating an open-plan kitchen-diner is one of the most common uses for side extensions, particularly in London, but the space is flexible enough to be adapted to your lifestyle. A home office, family snug or additional bathroom could all be factored into a side extension if that’s what you’re after.

Two-storey extensions (either side or rear) are the most comprehensive transformation, dramatically increasing your living space across both ground and first floors. Mark notes that if you’re planning on selling your property in the future, double-storey extensions with bedrooms get you real plus points from estate agents!

3. Brief with confidence 

If you’re clear on what you want from your new space and why you want it, you’re setting yourself up for success. Mark is less concerned about the format in which his clients brief him, than the clarity of their goals, which are everything. “Some people have photos, some might walk you around, some people write it out… it varies,” he tells us. But most importantly for an architectural designer (and anyone working on your extension) the end result is clear and achievable from the start – the details can come later. 

If you’re still grappling with ideas, start by considering long-term property value (adding an extra bedroom is desirable, for example), potential future requirements (maybe you need a nursery now, but how could you use that room in years to come?) and how the extension will integrate with existing spaces. 

4. Make sure you understand permitted development in your area

Mark’s number one piece of advice (which your designer should do for you) is research permitted development rights on your road. Permitted development allows you to make certain changes and upgrades to your house without having to get planning permission; what you’re allowed to do differs depending on your property type and area. 

“Usually you’d be aware if you’re living in a listed building or living within The Green Belt, but lots of people don’t know they’re in a conservation area, area of natural beauty, or area of special local character – a relatively new term,” explains Mark. As permitted development rights differ for these regions, not understanding what’s possible for your property from the off can not only lead to disappointment, but significantly delay extension plans.  “I’ve seen it happen so many times, where people haven’t realised what category their home falls into, and then have to change their plans,” he says. 

Another golden nugget of information Mark shared is that what you can do under permitted development you might not be able to get planning permission for. Because local councils have autonomy, they’re often stricter on rules that preserve neighbourhood character, so you might spend significant time and money putting in a planning application that gets rejected, which you didn’t even need to do in the first place. “That’s not to say that you couldn’t then add your extension via permitted development, you could still just get on with it, but you’ll have gone about it the hard way – many people just don’t know what you can and can’t do with permitted development,” Mark says. 

He notes that people often use their neighbours’ properties as a guideline for their extension, but this isn’t always the best way forward. Other properties might have not properly adhered to permitted development laws, and even seemingly small details like how many parking spaces you have or the size of your garage play into what you can build. 

Another top tip from Mark is to make sure you get a Lawful Development Certificate for your extension (which he always does for his clients). In essence, this is proof that the project is within legal boundaries, notifies neighbours who cannot object, and can be helpful when it comes to selling your property.

5. You don’t have to use the RIBA plan of work

If you’ve found yourself in a tizz about the RIBA plan of work and what it entails, it might be a relief to know that it’s not the only route forward. “You only use the RIBA plan when employing a registered architect,” Mark explains. “In my experience, it’s sometimes too comprehensive for residential projects.” That’s not to say that your build will be without structure: designers, contractors and structural engineers often have their own methodologies that they’ve developed with hands-on experience, which follow a similar structure and process, but are a bit less complex and can be more easily tailored to specific needs. 

6. Be patient: don’t underestimate the timeline

A typical extension project spans six to 12 months, broken into distinct phases. Design and planning usually take two to three months, obtaining permissions (if you need them) might require two to four months, construction typically runs for three to six months, with a final one to two months for finishing touches and addressing any final snags. 

Understanding this timeline helps manage expectations and plan accordingly. “People always want to move quickly, which I understand,” Mark says, “but when you rush, you make mistakes, we’re all human.” In his experience, projects that start badly – without clear budgets, goals, realistic timelines and requirements – are only going one way. It’s much better to get comfortable with the fact that extensions are a time-consuming job and focus on the end goal of creating a space you love, than try to get to the finish line too quickly and potentially compromise the result.

7. Trees, load-bearing walls and drainage can impact your plans

While load-bearing walls are elements that people might factor into their extension ideas (or have an awareness of), the impact of trees and drainage are things that Mark says often throw projects off course – and you just wouldn’t know unless you’ve done a similar project. 

Removing or modifying load-bearing walls (which you’ll typically do at least once with an extension) calls for structural calculations and proper support methods. Where your load-bearing walls are in your property impacts the extension possibilities and how to go about building it. As Mark says, “there’s no need for this to be an issue if you understand what’s possible from the start, but sometimes these aren’t factored in until later.”

When it comes to trees, it’s hard to tell just how far their roots reach. If they’ve grown under your home, the foundations could be damaged, so you might need council permission to remove them. Drainage is another thing that could easily be glossed over, but is actually very important: if drains need moving or have to be built over, this requires specialist knowledge and potentially council involvement too. These fears can be allayed from the outset by instructing a Tree Specialist Report, and a drainage survey.

Ready to see what you can do with your home? Get in touch with plansdesigned

A little more about Mark

Mark’s experience, plus extensive and diverse qualifications, allow him to deliver his clients beautifully designed homes they love at competitive prices. 

• Member of the British Institute of Architectural Technologists
• Member of the Chartered Institute of Building
• Member of the Architecture & Surveying Institute 
• Member of the Royal Institute of Chartered Surveyors
  

FAQs

How long does it take to build an extension?

Typically three to 12 months from initial design to completion, depending the complexity of the project.

Will an extension add value?

Most professionally designed extensions can add 10-20% to a property’s value, making them solid long-term investments.

How to finance a house extension?

Options include personal savings, remortgaging, home improvement loans and specialised renovation financing.

Do I need an architect?

You don’t need an architect, but it’s recommended to use an architectural designer or technician for complex projects to ensure design quality, regulatory compliance and the best use of space. 

Do I need a structural engineer?

A structural engineer is essential for making sure your extension is safe, particularly if you’re removing load-bearing walls or creating two-storey or structurally complex extensions.

Where can I live during construction?

It’s possible to stay at home and in designated liveable zones while your extension is being built, but the mess and disruption can be stressful. A phased construction approach (where the project is approached in stages) is another option; this minimises disruption in the short term but takes longer overall. Many people choose to rent a property nearby or stay with family or friends during their extension.

Find a home extension contractor with Thomas at The Page

Planning an extension doesn’t have to be overwhelming. At The Page, we specialise in matching homeowners with trusted, hands-on contractors who can bring visions to life at competitive prices – without sacrificing quality. We’ve already done the hard work of finding exceptional builders: we team up with Pagers not just for their skills but their approach to project management and communication.

Our renovation coaches understand both the practical and emotional aspects of extending your home, providing expert guidance at every step. We’ll help you understand the process, connect you with the right Pagers for your extension, and ensure you feel confident and supported from start to finish.