House Extensions in Bedfordshire, Hertfordshire & Cambridges
We design and build house extensions, including double storey extensions, across Bedfordshire, Hertfordshire, Cambridgeshire, and Buckinghamshire. As a family-run firm with over 30 years of experience, we are among the leading house extension builders, managing the full build process from groundworks to structure, roofing, and both first and second fix.
House Extensions in Bedfordshire, Hertfordshire and Cambridgeshire
An extension involves two separate decisions, how much you're adding, and where it goes. A single or double-storey build can sit at the rear, the side, or wrap around both. Conversions are a different job again: extra room without extra footprint
How Much You're Adding
Single-storey extensions
Single-storey extensions
Single-storey extensions
Adding on one floor, at ground level. The most common extension we build, usually to create a kitchen-diner or larger living space. Many fall under permitted development, meaning no full planning application, though building regulations always apply. One storey means shallower disruption: the roof stays intact above you and the existing bedrooms are untouched throughout.
Double-storey extensions
Single-storey extensions
Single-storey extensions
Building into the garden, at the back of the house. The position that takes glazing best — bifolds or sliders across the rear, roof lanterns carrying light back into the original house. How much garden you'll give up is usually the deciding factor. Our Upper Caldecote project is a rear extension with bifolds and skylights above.
Where it goes
Rear extensions
Wraparound extensions
Rear extensions
Building into the garden, at the back of the house. The position that takes glazing best, bifolds or sliders across the rear, roof lanterns carrying light back into the original house. How much garden you'll give up is usually the deciding factor. Our Upper Caldecote project is a rear extension with bifolds and skylights above.
Side extensions
Wraparound extensions
Rear extensions
Building into the plot beside the house, including the side return on a Victorian or Edwardian terrace. Narrower than a rear extension, but it turns a cramped galley kitchen into a proper room and leaves the garden intact. On semis there is often more usable width than owners expect.
Wraparound extensions
Wraparound extensions
Wraparound extensions
Rear and side together in an L-shape, the biggest ground-floor gain without moving house. Best suited to semi-detached and end-terrace properties with both garden depth and side width. A larger structural job, but it delivers the most floor area per pound of any single build.
Using space you already have
Garage conversions
Garage conversions
Garage conversions
Turning an attached or integral garage into habitable room, office, playroom, utility or bedroom. Because structure, foundations and roof already exist, this is typically the cheapest way to gain a room. The work is insulating floor, walls and roof to current regulations, replacing the garage door with a wall and window, and upgrading damp-proofing. Worth weighing against the parking and storage you lose.
Loft conversions
Garage conversions
Garage conversions
Converting unused roof space into a bedroom, en-suite or office. Which type suits depends on the roof: Velux where head height already works, a dormer where height must be added, hip-to-gable on a semi or end-terrace. Two things decide feasibility, head height at the ridge, and where the staircase lands. We check both at survey before anything is quoted.
Extension projects
Village property extension
An extension to a village property near Welwyn, matching the existing period detailing throughout.
Rear extension with bifolds
A rear extension with bifold doors and roof skylights, taken from groundworks through to completion.
Planning permission and building regulations
Most homeowners approach us unsure whether they need planning permission for their projects. Often, the answer is no. A significant portion of single-storey rear extensions, side extensions, and garage conversions fall under permitted development. This means you won't need a full application, but it's important to check the specific limits on depth, height, and the amount of garden space you can build over before designing your extension, especially if you are considering double storey extensions.
Full planning permission is typically required for double storey work, any projects exceeding permitted development limits, extensions to listed buildings, or properties in conservation areas. Even where permitted development applies, we recommend obtaining a Lawful Development Certificate from the council, as it serves as proof that the work was lawful when you decide to sell your home.
Building regulations approval is a separate requirement that applies to all projects, including extensions in Bedfordshire. This approval covers essential aspects such as structure, insulation, fire safety, drainage, and ventilation, and it is signed off by building control during the build rather than beforehand.
Additionally, a party wall agreement is necessary if you're building on or near a shared boundary, which is common in semi-detached and terraced properties, as well as in detached homes where the work is within 1 meter of the boundary. Our expert house extension builders can guide you through these regulations to ensure a smooth project.
What an extension costs
There is no single price for an extension, and any builder quoting one before seeing the property is guessing. Cost is driven by a handful of things, and the first three matter far more than the rest.
Footprint and storeys, floor area is the largest single factor, though a double-storey costs less per square metre than a single-storey of the same area. Ground conditions, foundation depth is decided by soil, trees and drainage, and it is the most common reason a quote moves. Glazing, bifolds, sliding doors and roof lanterns carry real cost, and specification varies widely.
After those: structural steelwork where walls come out, the level of finish you want, whether kitchens and bathrooms sit inside our scope or are fitted separately, and site access, a property where materials can only come through the house takes longer than one with side or rear access.
How an extension runs with us
1. Get in touch
2. Survey and scope
2. Survey and scope
Tell us about the project, what you need, where the property is, and roughly when you want to start.
2. Survey and scope
2. Survey and scope
2. Survey and scope
We visit the property, check access, drainage and structure, and confirm exactly what the build involves.
3. Itemised quote
4. Build and handover
4. Build and handover
A written, itemised quote before anything is agreed, with inclusions and exclusions set out plainly.
4. Build and handover
4. Build and handover
4. Build and handover
Work runs to the agreed scope with regular updates, building control sign-off and a final walk round.
Extension builders across four counties
Extension builders across four counties
Extension builders across four counties
Extension builders across four counties
Biggleswade, Sandy, Bedford, Ampthill, Flitwick, Shefford, Potton
Hertfordshire
Extension builders across four counties
Extension builders across four counties
Hitchin, Letchworth, Baldock, Stevenage, Welwyn
Cambridgeshire
Buckinghamshire
Buckinghamshire
St Neots and surrounding villages
Buckinghamshire
Buckinghamshire
Buckinghamshire
Milton Keynes and surrounding areas
Extension FAQs
Please reach us at info@jlsbpd.com if you cannot find an answer to your question.
Often not. Many single-storey rear and side extensions fall under permitted development, while double-storey work usually needs full permission. We confirm which applies to your property at survey.
A single-storey rear extension typically runs 10–14 weeks on site; a double-storey 16–22 weeks. Planning and building regulations approval sit ahead of that and add their own timescale.
In most cases yes. We seal the working area from the rest of the house and agree in advance when services need isolating, the disruptive stretch is usually knocking through at the end.
We hold full public liability insurance (£5m) and are accredited by the Federation of Master Builders and SafeContractor. Registered in England and Wales, company number 16751323.
Thinking about an extension?
Tell us about the property and what you're hoping to add with our expert house extension builders. We specialize in double storey extensions and can help you explore what's realistic for your project. We'll arrange a survey, discuss your ideas, and provide an itemised quote in writing, with no obligation either way, for extensions in Bedfordshire.
A family run construction company known for quality and professionalism.
Based in Biggleswade, Bedfordshire, we have built a strong reputation for delivering high-quality work across the surrounding areas.
Copyright © 2026 JLS Building & Property Development Ltd - All Rights Reserved.
Registered in England and Wales. Company No. 16751323.
+441767 235673
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