How Much Does a CLT Garden Annexe Cost in the UK?

Cross-laminated timber garden annexes represent one of the most substantial investments a homeowner can make in their property, and one of the most rewarding. If you are researching CLT garden annexe cost in the UK, the honest answer is that prices vary considerably depending on size, specification level, groundworks, and site access. What follows is a detailed breakdown of Celtio's three product ranges, an explanation of every cost layer you should budget for, and practical guidance to help you identify which model suits your situation.

What Is a CLT Garden Annexe?

A CLT garden annexe is a self-contained habitable structure built from cross-laminated timber panels, engineered wood formed by bonding successive layers of timber at right angles to each other. The structural result is a panel with exceptional strength-to-weight ratio, certified fire resistance, and superior thermal and acoustic performance compared with conventional timber frame construction.

Celtio's Celtic range takes this material and applies it in a fully prefabricated modular format. Each unit is manufactured off-site as a complete, finished module and delivered to site on a specialist lorry. It is then crane-lifted directly onto prepared foundations and connected to utility services, typically within a single day. The structure does not arrive in panels for on-site assembly. It arrives as a building.

That distinction matters commercially as much as it does technically. Because the module is crane-lifted as a single intact unit, it can, if circumstances change, be lifted back onto a lorry and relocated to another site without any dismantling whatsoever. The building can move with you, or move with your planning decisions. This is a defining characteristic of Celtio's product that no conventional timber frame or brick-built garden annexe can replicate.

2026 Celtio Price Summary: Celtic, Celtic Mini and Celtic+

The table below presents the three ranges available from Celtio, with price indications derived from the March 2026 product price list. Figures shown are inclusive of a 25% supply margin and are net of VAT. They reflect the base specification at the standard external finish (vertical spruce boarding facade with standing seam metal roof) and exclude the one-off bespoke architectural and structural design fee, which applies only to individually designed projects. Transport to site, crane hire, groundworks, foundation preparation, and utility connection are quoted separately on a project-by-project basis.

Cost Breakdown: What You Are Actually Paying For

Garden annexe pricing is frequently presented as a single headline figure, which obscures a considerable amount of useful information. The Celtio price list structures costs across five distinct stages, each of which reflects genuine work and material specification decisions that affect the final building.

Stage I: Design and Structural Engineering

For bespoke or individually specified projects, a one-off design fee applies. This covers architectural drawings specific to your site, CLT structural calculations signed off by a structural engineer, and internal installation designs. For UK buyers, these documents form part of the planning or compliance submission. This fee is not included in the price indications in this article, as it applies only where an individual design package is commissioned.

Stage II: Shell Stage

The shell stage includes the CLT and glulam structural module itself, together with triple-glazed PVC-U external windows and doors. For the Celtic Mini, the CLT structure accounts for the largest single cost component at this stage; similarly for the Celtic and Celtic+. These figures reflect non-visual grade CLT, the material is structurally identical to visual grade but the surface finish is not exposed in the final building. An upgrade to visual-grade CLT, where timber surfaces are left exposed rather than lined with plasterboard, is available as an optional add-on.

Stage III: Developer Stage

The developer stage brings the structure to a standard where internal fit-out can proceed. For UK-specification units, this stage is mandatory and includes GK plasterboard on a mineral wool frame across all internal walls and ceilings, a requirement for compliance with BS 3632, together with floor insulation and levelling layers, full electrical and telecommunications installations, plumbing and drainage, a distribution board, a concealed frame for a wall-hung WC, and an 80-litre electric hot water boiler.

Stage IV: Turnkey Finish

The turnkey stage is where the building becomes a finished, habitable space. It covers wall-mounted MVHR heat recovery ventilation units (mandatory under Approved Document Part F for UK buildings), electric underfloor heating throughout all living areas, full internal decoration, complete bathroom waterproofing with full-height tiling and sanitary ware, built-in kitchen units with sink and tap, fitted wardrobe and storage, and built-in appliances including an induction hob, extractor, mini fridge, and dishwasher. By the end of this stage, the building requires only its utility connections to be occupied.

Stage V: External Finish

The base external specification includes a PIR insulation board beneath a standing seam metal sheet roof, a vertical spruce timber facade with mineral wool backing, an EPS-insulated plinth finished in Rockpanel Colours composite panels, and integrated structural transport lifting points, the crane attachment points built into the module's structure from the outset, which make both delivery and future relocation possible.

Alternative facade specifications are available. A full Rockpanel Colours composite cladding facade, a charred timber option (exclusive to the Celtic Mini), and an upgrade to aluminium external joinery are all available as priced additions.

What Additional Costs Should You Budget For?

The price indication covers the manufactured unit. Getting that unit onto your site and connected to services involves additional costs that are quoted individually because they depend heavily on site-specific conditions.

•      Foundation and groundworks: The module requires a prepared concrete pad or screw pile foundation. Cost depends on soil conditions, ground levels, and drainage requirements. Budget from £3,000 upwards for straightforward sites, with more complex ground conditions adding considerably to this figure.

•      Transport: The module travels from the manufacturing facility on a specialist low-loader. Transport cost is quoted per project based on distance and access route.

•      Crane hire: Because each Celtic module is a single, fully fabricated unit, a crane is required for installation. The weight and structural integrity of CLT construction mean the building cannot be walked in on rollers or assembled in situ. Crane hire for a one-day lift is quoted individually and the requirement should be factored into any site access assessment from the outset.

•      Utility connections: Mains water, drainage, electricity, and any broadband cabling must be extended to the module from the host property or direct mains supply. Costs vary by distance and existing infrastructure.

•      Planning or compliance documentation: Where a planning application is required, professional fees and application costs apply. Where the caravan legislation route is used, a compliance certificate may be required.

Which Celtio Product Is Right for You?

Celtic Mini: from ~£54,000

The Celtic Mini measures 7.4 x 3.8 metres, providing 19.2 square metres of usable floor area. At a price indication from approximately £54,000 net before transport and groundworks, it is the most accessible entry point into CLT modular annexe ownership. Available in three configurations, compact guest accommodation or studio living, standalone sauna building, and home office module, the Celtic Mini suits homeowners who need a functional, well-insulated, year-round space without the footprint of a full annexe.

Celtic: from ~£69,000

The Celtic measures 9.7 x 3.6 metres, with 26.25 square metres of ground-floor usable space and an optional 6.7 square metre mezzanine accessed via an integrated ladder and balustrade. The ground floor accommodates an open-plan kitchen and living area, a separate bedroom, and a full bathroom. Price indications start from approximately £69,000 net. The Celtic is the most widely specified configuration for multigenerational living, providing sufficient separation and amenity for a single adult or couple. Available with either a pitched or low-slope roof profile.

Celtic+: from ~£81,400

The Celtic+ measures 12 x 3.6 metres and provides 32.7 square metres of usable floor area across two private bedrooms, an open-plan kitchen and living area, and a full bathroom. At a price indication from approximately £81,400 net, it is the most capable configuration in the range and suits situations where two adults require independent accommodation, or where longer-term flexibility across multiple occupants is a priority.

CLT Garden Annexe Costs and Property Value

A permanently installed, professionally built, and fully habitable garden annexe adds measurable value to a residential property. Industry estimates consistently suggest that high-quality ancillary accommodation adds between ten and twenty per cent to overall property value, depending on location and the quality of the structure. CLT construction is well regarded by surveyors and mortgage lenders because of its structural certification, thermal performance, and durability record.

Celtio's Celtic range is built to UK BS 3632 residential standard, which sets minimum requirements for thermal performance, acoustic insulation, structural integrity, ventilation, and fire resistance. Where a unit meets BS 3632, VAT zero-rating on the supply may be applicable, subject to HMRC confirmation and the specific circumstances of the installation. This is a matter for specialist VAT advice, and Celtio recommends confirming eligibility with a qualified adviser before relying on it in any financial assessment.

Frequently Asked Questions

Do I need planning permission for a CLT garden annexe?

Planning permission requirements depend on the specific circumstances of your site and the intended use of the structure. Celtio's Celtic modules are dimensioned to comply with the Caravans Act definitions, a route to installation that does not require a conventional planning application in many cases, provided the site meets the relevant legal criteria. This is a technically and legally specific area, and Celtio's team works through the appropriate compliance route for each project individually. Independent legal advice is recommended where the position is unclear.

How long does installation take?

Once foundations are in place and the unit has been manufactured, the on-site installation process typically takes a single day. The module arrives complete, is crane-lifted onto the prepared foundation, and is secured and weather-tight by the end of the same working day. Utility connections are completed as a separate trade activity. Total project timeline from confirmed order to habitable unit is typically six to ten weeks.

Can the annexe be relocated in the future?

Yes. Because each module is manufactured and delivered as a single integral unit, with structural crane attachment points built into the construction from the outset, it can be crane-lifted off its foundation, loaded back onto a lorry, and transported to a new site. No dismantling is required. No structural compromise occurs. The building moves as a building. For buyers who anticipate changes to their property or family situation, this redeployability represents a meaningful risk reduction compared with any permanent built structure.

Is financing available for a CLT garden annexe?

Several financing routes are available to UK buyers, including secured home improvement loans, further advance borrowing against existing mortgage equity, and specialist garden annexe finance products. Because a Celtio module meets BS 3632 residential standard and is installed on a permanent foundation, it is generally regarded more favourably by lenders than a temporary or non-habitable garden room. Celtio can provide specification documentation to support any finance application.

What warranties are included?

Celtio's Celtic range carries a ten-year structural warranty. Windows, doors, and roofing elements carry manufacturer warranties in addition. The MVHR ventilation units and electrical installations are covered under the relevant manufacturer and installation guarantees. Full documentation is provided at handover.

Request a Quote or Speak to the Celtio Team

Every garden and site is different. Pricing depends on your chosen model, specification level, site access conditions, and groundworks requirements. The figures set out in this article provide a reliable framework for budgeting, but an accurate cost requires a site-specific assessment.

Visit celtio.co.uk to request a detailed quote, download the full product brochure, or arrange a consultation with the Celtio team. You can also call directly on 0800 955 2025 or email hello@celtio.co.uk.