Built to last, not just to look good
A granny annexe from Mobile Annexe Ltd is built to genuine residential standards — not a garden shed with a bed in it, and not a basic park home. Every annexe we build is designed to feel, perform and last like a proper home, because that's exactly what it is.
A genuine timber frame
Every Mobile Annexe is built on a C24-grade kiln-dried structural timber frame — the same graded, stress-tested timber standard used in quality timber-frame housing, not a lightweight or unrated softwood frame.
The frame is specified according to where it's doing the most work:
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8" x 2" timber to the floor and ceiling joists, giving a genuinely solid, robust structure underfoot and overhead
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6" x 2" timber to the external walls, providing both structural strength and generous depth for high-performance insulation
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4" x 2" timber to internal walls — sized appropriately for non-load-bearing partitions, keeping internal layouts efficient without compromising strength where it's needed
Kiln-drying the timber before use reduces moisture content and helps prevent the warping, shrinkage and movement that can affect lower-quality or "green" (undried) timber over time — meaning a more stable, long-lasting frame.
Wall build-up: layer by layer
From the outside in, here's exactly what makes up an external wall on a Mobile Annexe:
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External cladding — your choice of cement board, timber cladding, render, or brick slip, depending on the look you want and how you'd like your annexe to sit alongside your main house
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50mm x 25mm battens, creating a ventilated cavity behind the cladding
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House wrap membrane, providing a breathable weather barrier
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18mm OSB board, forming a strong, rigid sheathing layer across the timber frame
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12.5mm plasterboard
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Plaster finish
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Paint, in your chosen colour
This layered approach — cladding, ventilated cavity, breather membrane, structural sheathing, and a proper plastered internal finish — is the same standard used in quality timber-frame housing, giving your annexe genuine weather resistance, breathability, and a smooth, ready-to-decorate internal finish.
Insulation: warm, and genuinely fire-safe
We insulate every wall cavity with 125mm Knauf DriTherm 37 mineral wool insulation — a high-performance product that keeps your annexe warm in winter and cool in summer.
Fire safety is a big part of why we specify it. Mineral wool insulation like DriTherm is non-combustible, achieving the highest tier of fire safety classification available — a genuinely different standard to foam-based insulation products, which are rated as flammable and can contribute significantly to fire spread. For a home your family will actually live in, we don't think that's a trade-off worth making.
Roof construction: your choice of flat or pitched
We offer two roof options, so your annexe can suit your personal taste and how it sits alongside your main house — and both are built and insulated to the same high standard.
Flat/pent roof. Constructed using the same 8" x 2" C24-grade kiln-dried structural timber used in our floor joists — a genuinely robust roof structure, not a lightweight afterthought. Firring pieces are added across the joists to create a gentle fall, ensuring rainwater runs off properly rather than pooling. The structure is decked with 18mm OSB board, then finished with a fibreglass (GRP) roof covering — a genuinely watertight, low-maintenance roofing system with none of the seams or joints that can fail on felt or traditional flat roofing materials.
Pitched roof. Built to a 20-degree pitch as standard — increased to 31 degrees where solar panels are required, giving the correct angle for effective solar performance. The roof is finished with a 170g Strotex vapour-permeable membrane, allowing moisture to escape from within the roof structure while keeping the elements out, and covered with Envirotile lightweight plastic roof tiles — a durable, low-maintenance tile that gives the authentic look of natural slate without the weight or cost.
Whichever option you choose, both our flat and pitched roofs are fully insulated, keeping your annexe genuinely warm and energy-efficient year-round.
Foundations: the Swift Plinth Plus system
Every annexe sits on a purpose-built Swift Plinth Plus foundation system — a genuinely engineered ground-bearing solution, not a basic concrete pad.
The system is built up in four parts:
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Swift Grid — a load-bearing grid that improves drainage around the base of the foundation, helping to manage groundwater rather than trap it
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Base Stone — interlocking stones that prevent sideways slippage, keeping the whole foundation securely locked in place
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Top Stone — a pre-cast, reinforced stone that bears the full weight transferred down through the steel bracket above
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Bracket — a heavy-gauge steel bracket offering up to 50mm of adjustment, allowing the frame to be precisely levelled regardless of minor ground variation
Each foundation pad is engineered to hold up to 2.5 tonnes, giving your annexe a genuinely solid, reliable base — without the disruption, cost, or time of traditional concrete strip foundations.
Glazing: your choice, built to your standard
Doors and windows are entirely down to you — we offer double or triple glazing, in UPVC, aluminium, or timber frames, so your annexe's windows and doors can match your taste, budget, and how you'd like it to sit alongside your main home.
Built to full building regulations standard
Every Mobile Annexe is built to conform to all relevant building regulations — not just the minimum required to satisfy the Caravan Act's classification rules. While most of our annexes are built under the Caravan Act (meaning you'll typically only need a Certificate of Lawful Development rather than full planning permission), the construction itself is held to the same genuine standard as a permanent home, so you're never trading legal simplicity for build quality.
Built for flexibility — on-site or in our workshop
Unlike providers who rely entirely on off-site factory manufacturing, our wall panels are constructed either on-site or in our own workshop, then joined together on-site to form your completed annexe. This gives us genuine flexibility on every project — and gives you real input throughout the build, rather than receiving a pre-decided factory unit with little room for changes once it's underway. For a bespoke project like a granny annexe, being able to adapt to your specific plot, garden and preferences as the build progresses is a real part of what makes it genuinely yours.
Every detail considered
From the groundworks up, our full-time, in-house project management team oversees every stage of your build — not subcontracted out and forgotten about. Your dedicated site foreman and customer liaison are with you from foundation to final handover, typically over 12–16 weeks, keeping you informed at every step.
Why this qualifies under the Caravan Act: the legal detail
Every annexe we build is designed to satisfy the three legal tests that determine whether a structure qualifies as a "caravan" under the Caravan Sites Acts 1960 and 1968 — the size test, the construction test, and the mobility test.
The size test. Twin-unit caravans can measure up to 20 metres in length and 6.8 metres in width — the same maximum dimensions behind our 136m² size limit.
The construction test. Our annexes are built as two sections, joined together on-site as the final act of assembly. It's a common misconception that a caravan must be delivered to site already built in sections — it doesn't. This has been confirmed at appeal on multiple occasions, including a case where an inspector found no requirement in the legislation that the two sections be constructed away from the site where they're joined, provided there's a genuine final act of assembly bringing them together. Our approach — building wall and roof panels that are joined on-site — follows this established, tested legal position.
The mobility test. Rather than being physically fixed to the ground, every annexe rests on our Swift Plinth Plus foundation system, resting under its own weight rather than being permanently attached to the land. This has been confirmed at appeal to mean a structure retains the necessary "capable of being moved" status — connections to services like water and electricity can be disconnected in minutes, and the completed structure remains capable of being lifted and transported, even if that's never actually done in practice. Because our annexes are engineered so that a mobile crane could lift and move each section if required, this test is satisfied by design and structural engineers, not just on paper.
Together, these tests are why our annexes qualify as caravans in law — meaning most projects need only a Certificate of Lawful Development rather than full planning permission. We manage this application on your behalf as part of every project, with a 99% success rate securing approval for our clients.
See it for yourself
Photos only tell you so much about build quality. If you'd like to see a completed annexe in person, or speak to a past client about their experience, we're happy to arrange this as part of your free consultation.
Quality Summarised
Every annexe we build includes:
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Full 125mm Knauf DriTherm 37 mineral wool insulation, keeping your annexe warm in winter and cool in summer
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Double or triple glazing, in UPVC, aluminium or timber frames, to suit your preference
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Electrics and plumbing installed and certified by qualified, registered tradespeople
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Heating tailored to your preference — usually efficient oil-filled electric radiators as standard, though gas, ground source, air conditioning, or any other heating system can be accommodated if that's what you need
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Manufacturer warranties on windows, doors, cladding and roofing materials
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Our own 10-year structural guarantee on the completed build
FREE QUOTATIONS
Arrange a free planning consultation to realise your dream of a granny annexe or garden annexe by calling Mobile Annexe Ltd today on 07770 365734.