Forest Xtend 4.0+ Insulated Garden Office 4.05m x 3.42m (97mm)

Product code:

From £12,699.00

To £14,858.00

The Xtend 4.0+ is an industry-leading garden office, using revolutionary SIP construction to create a fully-insulated garden room which will remain a pleasurable space to work from home throughout the seasons.

  • Constructed from revolutionary Structural Insulated Panels (SIPs) for thicker, better, year-round insulation
  • 97mm SIP walls with attractive 15mm tongue and groove external cladding plus further internal cladding for an incredible 177mm thickness
  • 108mm SIP floor and 122mm SIP roof for top-to-bottom, superior insulation (images shown with laminate flooring added)
  • Double-glazed double doors with 7-point multi-lock; 2 full-length, double-glazed windows
  • 15-year structural guarantee
  • Other sizes available here
Size & Specification

Garden Office Only
Forest Xtend 4.0+ Insulated Garden Office 4.05m x 3.42m (97mm)   + £12,699.00
Optional Office Installation

Please ensure a solid, level base is constructed BEFORE your installation date. Installation does not include electrics or other accessories. Please see the installation section below for more details.

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  • Accessories
  • Specification
  • Description
  • Trustpilot Reviews
  • Xtend 4.0+ Features
  • From The Supplier: Forest Garden
  • Xtend Garden Buildings Installation
  • Delivery


Brand Forest Xtend
Cladding Tongue and Groove
Construction Type SIPs panels with external Tongue & Groove cladding/internal MDF cladding
Door Type Double Doors
Eaves Height 2400
Size 13' x 11'
Internal Depth 2680
Internal Width 3670
Opening Windows 2 windows
Ridge Height 2500
Roof Covering EPDM Rubber Membrane
Roof Style Pent
Material Wooden
Guarantee 15 years
Depth at Base 3040
Depth including Overhang 3420
Width at Base 4030
Width including Overhang 4050
Door Height 1950
Door Width 1600
Glazing Material Double Glazing
Locking System 7 Point Multi-Lock With Ultion Three Star Security Barrel
Floor Material Structural Insulated Panels (SIPs)
Floor Thickness 108
Roof Material Structural Insulated Panels (SIPs)
Roof Thickness 122


The Forest Xtend 4.0+ Insulated Garden Office is a modern garden room designed to work with your busy lifestyle. As well as the ideal home office, this versatile garden building also makes an excellent year-round family space or hobby room.

Revolutionary Construction

As an economic alternative to a home extension, this premium garden room is constructed from pre-cut, modular structural insulated panels (SIPs). Many new homes are now being built this way as SIPs offer ultimate insulation, are energy efficient and are robust. Unlike many other brands who offer just insulated walls, the Xtend range features SIP construction for the roof and floor too. The result is a fully insulated garden room from top-to-bottom.

The incredible level of insulation also ensures that the home office will not be subject to damp, will be cool in summer, and will remain warm in winter - making it a pleasurable environment for working from home throughout the seasons.

Each wall SIP consists of 97mm insulation sandwiched between two sheets of OSB (oriented strand board). The SIPs are completed and further insulated by both external and internal cladding. The internal cladding is MDF with a vertical beaded profile for a sophisticated appearance. It is primed ready for painting. The external cladding is high-quality, kiln-dried 15mm tongue and groove timber. As well as offering a stylish finish, tongue and groove is known for its weathertight structure. This combination of SIP, cladding and the cavities between gives the walls an incredible overall thickness of 177mm.

Floor and Roof

For increased insulation, the 108mm floor and 122mm pent-style roof are both built from SIPs too. The floor is left uncovered, so can be finished with carpet, laminate or other flooring according to personal taste. The interior of the roof is finished with pre-primed interior cladding ready for painting. Outside, the roof features a market-leading, black EPDM weatherproof roof covering, with an exceptional 50-year life expectancy. This rubber roof covering will not split or crack, does not support moss or algae growth, is UV-stabilised and is completely maintenance free. Anthracite grey facias complete the modern finish.

Forest do not offer an electrical service. However, the 28mm wall battens and 45mm roof battens are designed to leave enough space to add electrical wiring and recess lights respectively. Electrical work should always be undertaken by a qualified electrician.

Doors and Windows

Entry to this garden office is gained via double-glazed, UPVC double doors, which are secured by a 7-point multi-lock to keep expensive equipment safe. Two additional full-length, double-glazed windows ensure the interior benefits from an abundance of natural light. The top half of both windows opens for ventilation. The doors and windows all have grain-effect grey outer and smooth white inner frames, making for an exceptionally attractive appearance.

Options Available and Assembly

This top-of-the-range insulated garden room is available in 3 sizes and with 3 different door/ window configurations.

Fixtures, fittings, exterior facias and a step-by-step installation guide are all included for a simple assembly. Forest do offer a professional installation service carried out be expert fitters.

Energy Efficiency and Care Instructions

This garden home office has an energy efficiency rating of 78 (category C) and has the potential to reach 82 (category B), with the addition of a photovoltaic solar panel.

The outdoor office is delivered untreated. As such, the exterior should be treated upon assembly and again each year. Forest Garden recommend that it is cared for with one of the following: Sikkens Cetol HLS Plus, Restol Wood Oil, Timmersol ETS Double Protectant, Osmo Country Colour or Osmo Natural Oil Wood Stain. The internal cladding comes with primer applied, so can be painted in any colour using standard interior emulsion paint.


Delivery is free to most UK addresses and free pick-a-day delivery is available to many areas. Some remote postcodes may incur surcharges. Please see the delivery information for further details.

  • 15-year structural guarantee
  • UK-made from revolutionary Structural Insulated Panels (SIPs)
  • 15mm tongue and groove external cladding and MDF interior walls
  • 108mm SIP floor and 122mm SIP roof for top-to-bottom insulation
  • EPDM weatherproof roof covering, with a 50-year life expectancy
  • Double-glazed, UPVC double doors with 7-point multi-lock
  • 2 full-length, double-glazed windows
  • Available in 3 sizes and with 3 different door/ window configurations
  • Energy efficiency rating 78 (C) - potential 82 (B)
  • Free delivery to most UK addresses

Trustpilot Reviews

Xtend Garden Building 4.0+ Features

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From the Supplier: Forest Garden

"We're proud to be the largest and most trusted manufacturer and distributor of garden timber products in the UK" - Forest Garden

Forest Logo

Making British Gardens Great

At Forest, we know a fair bit about fences. And sheds for that matter. In fact, we've been one of the UK's most trusted manufacturer and distributor of quality wooden garden products for over 50 years.

With two sawmills in the UK processing locally-sourced timber, and a Head Office in Hartlebury, Worcestershire, you’ll find our range of garden buildings, fencing, decking and decorative structures for sale across many sites online and in over 3,000 retail stores. From the forest to the doorstep, we are involved every step of the way to ensure we bring you the best product and service.


Home Grown

We're proud to say that the majority of the timber we use is cut in our very own sawmill from trees grown in the UK.

Not only that, but our range is certified by the Forest Stewardship Council and the majority of our products carry the FSC logo - so you know that our timber is ethically sourced from well-managed, sustainable forests that are replanted for future generations.

(Certification number TT-COC-001790, Issue date 5 March 2015)

Made in the UK

Well made in Britain

The majority of the timber we use is grown and felled from managed UK forests across Scotland and Wales. After felling, the timber is cut and processed in one of our nearby sawmills and taken to our manufacturing operation in the heart of Britain. It is here our skilled workforce proudly produce the widest range of garden buildings, fencing and structures available in the UK.

All of the products that we sell are dispatched from this site. The final part of their journey is managed by our dedicated delivery department who are responsible for distribution direct to customers' homes by our fleet of vehicles.

Thrive logo

Helping Everyone Thrive

We like to do our bit for charity and are proud to support Thrive. Thrive uses gardening as therapy to bring about positive changes in the lives of people living with disabilities or ill health, or those who are isolated, disadvantaged or vulnerable.

Forest Installation

Forest Installation 


Forest Xtend Garden Buildings Installation

Forest Garden provide their own expert fitters to install your Xtend Garden Building. With dedicated experience installing Forest's Xtend Range, they will provide a professional service that is second to none. They aim to carry out installation within 15 - 20 working days of the delivery of the garden office and it is usually quicker than this estimate.

After your order, Forest Garden will send you a detailed document covering the requirements of a suitable base for a Xtend Garden Building.

Forest Garden do not provide a service for the preparation of the base area or for installation of electrics, wi-fi, solar lighting or plastic bases. If you require assistance with these, we recommend the use of for locating a reputable tradesperson in your area.

If you choose to install the garden office yourself, you can use the detailed guide which includes step by step images to help you. Forest Garden also have a Technical Helpline if you have any questions along the way.

If you are using an independent tradesperson to install your garden building, please allow enough time for the garden building to be delivered and the parts inspected before booking your tradesperson.

Please inform customer service BEFORE you order, if you believe there are restrictions which could impact the delivery and installation of the garden room.


Xtend Delivery

Xtend Delivery 

We offer FREE Delivery to the majority of mainland postcodes but some areas may incur an additional carriage charge (see below). Please use our postcode checker further up the page (below the add to basket button) or within checkout to check delivery is available to your area.

£49.99 - BH, BN, CA, CT, DH, DL, DT, EX, IP, LA, LL, ME, NE, NR, PL, PO, SA, SO, SR, TA, TN, TQ, TS

£99.99 - TR

£199.99 - AB, DD, DG, EH, FK, G, IV, KA, KW, KY, ML, PA, PH, TD


Your delivery will be made between 8am and 6pm on your pre arranged delivery date; our delivery team will contact you to book via phone.

Our service is a Kerb side pallet drop delivery by a crane off-load vehicle. Unfortunately we will not be able to unpack your Xtend Garden Building and take your goods to the back of the property. We typically plan to deliver to the front garden or drive.

Xtend Garden Buildings are delivered in two packages, flat packed, direct to your home and supplied with a comprehensive set of instructions to help you get started.

Please Note - delivery is NOT available to the following locations: Isle of Man, Isle of Wight, Isles of Scilly, Northern Ireland, Channel Islands, Scottish Islands.


Extended Lead Times: Due to the impact of the pandemic on manufacturing across the nation and on the importation of raw materials, alongside the highest demand for gardening products the industry has ever seen, some lead times for products are now longer than usual. Manufacturers are working hard to keep supplying customers. Thank you for your patience.

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