Fixed price · Planning handled · Cockfosters based
Design, planning and build, handled end to end by one team. Extensions, lofts and whole-house transformations — built by Pine Grove Design & Build, 25+ years in North London.
The Extension Company is the extensions arm of Pine Grove Design & Build Ltd — the same team designs your extension, gets it through planning and builds it. No sub-contracted middleman, one point of contact throughout.
Lines open every day, 9am–8pm. If we're on site and miss you, we call back the same day.
Professional home extension for every project
Residential extensions, renovations, and new home designs tailored to your vision
Conservation areas, wealth-belt streets and characterful stock — sensitive design that wins approval
Expert guidance through extension planning permission, extension building control, and permitted development
Get your extension project completed in 3 simple steps
Tell us about your project vision and requirements through our simple online form
Our house extension specialists create detailed designs and handle all extension planning applications
Complete your extension project with professional drawings and approvals
What clients say
“The Extension Company built a 5m single storey rear extension on our Victorian terrace in Crouch End. Went down the permitted development route so no planning application needed, which they explained up front and saved us months. Kitchen-diner now opens onto the garden with sliding doors. £87k all in, matched the quote.”
“From the initial site visit onwards to my property in Barnet, we felt that the team actually listened to what we wanted. They suggested a few changes to the layout that we hadn't considered, and those changes have made a huge difference. Would definitely use again and recommend”
“Used The Extension Company for a loft conversion on our semi detached in Barnet and couldn't be happier. They explained what was possible at the first visit, sorted the planning side, and gave a fixed price with no surprises at the end.…” read the full review
“We spoke to a few companies before deciding to go with The Extension Company and they just seemed the easiest to deal with. No pushy sales, just honest advice. They kept us updated and the whole process was a lot less stressful than we expected”
“Really impressed with the entire project. The side extension and conservatory have been finished to a very high standard and the team made the process straight forward and stress-free. It’s transformed the space and added so much value, would definitely recommend.”
“Brilliant experience with the extension company, transparent from the word go. Completed a double story extension for me and I couldn’t be more pleased. Thank you”
“Couldn't be happier with our loft conversion. We were expecting the build to be stressful, but it was actually a lot easier than we'd imagined. The lads were friendly, worked hard, and the quality of the finish is spot on.…” read the full review
“I got the extension company to do a bathroom refurb for us a January they done a great job they managed to fit in a large shower as well as a bath”
Reviews shown word for word from our Google profile. Read all 16 reviews on Google
Professional house extension specialists across London
Quick design decisions and efficient project delivery. No delays, no excuses.
Transparent fees and competitive pricing. No hidden charges.
Dedicated house extension specialists guide you through every step of the design process.
Experienced team with strong track record of extension planning application approvals.
Consistent success in getting extension planning applications approved across London and Hertfordshire.
Residential, commercial, extensions, renovations - we design it all.
Design, planning and build under one fixed price — across North & South London
Rear, side, wrap-around and double-storey extensions, designed and built end to end.
Open-plan kitchen and family space, with the kitchen move engineered on the drawings before pricing.
The Victorian terrace classic — filling the side return to widen the ground floor.
The schemes that change what a house is, upstairs and down.
Dormer, hip-to-gable and mansard conversions — the classic winter build.
Brick piers, roof lanterns and all-year rooms, built to full building regs.
Insulated garden buildings designed around permitted development limits.
An extra room out of dead space, usually with no planning application.
Dig-outs and cellar conversions, with the structural work handled in-house.
Self-contained and multi-generational annexes, planning included.
Full renovations under one fixed, itemised price.
Planning and building-regs drawing packs, priced against what councils approve.
Where we work
Forty area guides built on real council planning data — from our Cockfosters base across Enfield, Barnet and the Hertfordshire border. And we now build south of the river too.
Get answers to common home extension questions
A single-storey rear extension typically runs £60,000 to £120,000+ in North London depending on size, glazing and finish. We quote a fixed, itemised figure after a free site visit — or design yours in the Extension Builder for an honest guide price in 60 seconds.
Many single-storey extensions fall under permitted development, so no full application is needed. Where permission is required, we design to what your council demonstrably approves and handle the whole application for you.
Design and planning usually take 8-12 weeks, and a single-storey build typically 10-16 weeks on site. We agree a programme with you before work starts and manage it end to end.
The Extension Company is the extensions studio of Pine Grove Design & Build Ltd — the same team designs, manages and builds your project. Nothing is passed on, sold on, or subcontracted to strangers.
Yes. You get a fixed, itemised figure before you sign, and the number you sign is the number you pay — it only changes if you change the specification.
Everything: design and drawings, the planning application, building control, structural engineering, the build itself and project management — one team from first sketch to handover.
Most London extensions touch a shared wall, so yes — we advise on party wall notices early and work alongside appointed surveyors so the agreement never holds up your build.
All our designs comply with UK building regulations, and we produce the technical drawings and manage building control sign-off as part of the fixed price.
Yes — loft conversions, wrap-arounds, double-storey extensions and whole-house transformations across North London and the Hertfordshire border.
We can usually do your free site visit within days, design starts straight after, and build slots are typically booked 6-10 weeks out. Call 020 3051 9430 to check current lead times.
Have more questions? Get personalized answers below.
Get a QuoteTell us about your needs and get a personalized quote in minutes
Practical advice on costs, planning permission, and design from our house extension specialists.
A lawful development certificate is the council's written confirmation that your extension is legal. Here's what each type costs in 2026, how the 2024 rule change killed the 4-year rule, and how to apply properly.
Read Guide
Most extensions built "without planning permission" are perfectly lawful — that is what permitted development is. Here is what happens when a project falls outside it, and how to fix unauthorised work properly.
Read Guide
Most garden buildings are permitted development — until one of six conditions trips you up. The 2.5m boundary rule, the 50% rule and the "incidental use" test catch people constantly. How each one works.
Read GuideGet answers to the most common questions about home extension
Our expert team is here to help you with your home extension needs
Explore More House Extension Services
Discover our comprehensive range of house extension solutions across North & South London
Quick Access
Our Services
London Areas We Serve
Expert Team Available
Call now to speak with our expert team. Get instant responses and professional guidance for your extension project.