TRJ CONSTRUCTION · HOW IT’S MADE

How Our SIP Garden Rooms
& Granny Annexes Are Built

From the foundations beneath your feet to the roof above your head. See the materials, methods and choices that go into a TRJ building.

See how we build ↓

Understand your building, from the ground up.

At TRJ Construction Ltd, we build SIP garden rooms and granny annexes around the site and the way you intend to use the space. This guide explains the foundation options, structural panels, roofs, external finishes, interiors and utility connections, using photographs from our construction portfolio.

STEP 01

Foundations and base systems explained.

The foundation transfers the building’s load into the ground. The base supports the floor above it. They work together, but serve different purposes.

Screw pile foundations

Screw piles are driven into suitable ground to support the building, avoiding a large continuous concrete slab where the design allows.

  • Reduced excavation and ground disturbance
  • Useful where access is restricted or the garden is uneven
  • Fast installation once the site and design are confirmed
  • Can reduce concrete use compared with a raft

Screw piles can be installed up to 1.2m deep in suitable conditions. The required depth, pile layout and load capacity must be confirmed for the actual ground and building.

SIP shell on an elevated base — the support system is designed for the site.

Reinforced concrete raft foundations

A reinforced concrete raft creates a continuous structural base. It can be suitable for larger annexes or sites where the ground and engineering design require it.

  • Excavation and preparation to the specified levels
  • Steel reinforcement within the concrete
  • Damp-proof membrane and moisture protection
  • A level base designed around the proposed building

The thickness, reinforcement, drainage and ground preparation are specified for the project and relevant approval requirements.

Steel reinforcement and membrane before the concrete raft is poured.

Steel frame base construction

A fabricated steel frame forms a supporting platform for the SIP floor. It is a base system, supported by the foundations specified for the project.

  • A coordinated frame and SIP floor layout
  • Support points agreed with the structural design
  • A level platform for the insulated floor
  • Connections and protection detailed before installation

The photograph shows a fabricated steel frame before installation.

Fabricated steel frame, ready for the next stage of the build.

STEP 02

The floor, walls and roof: one SIP structure.

Structural Insulated Panels combine load-bearing board faces with a solid insulation core. They form the insulated shell, rather than insulation being added later between studs.

What is inside a SIP panel?

  • Structural OSB board faces — 12mm boards in our stated specification
  • A solid polyurethane (PUR) insulation core
  • Precision joining splines to connect the panels
  • Sealed joints and openings to control air leakage

Our low-height building specification

For low-level designs under 2.5m, our specification includes 150mm SIP roof and base panels and 125mm structural wall panels. The complete structural and thermal specification is confirmed for each building.

Good panel connections, sealing and detailing reduce gaps and cold bridging. Glazing, ventilation and heating also contribute to comfortable year-round use.

Read more about our SIP panels ↗
SIP wall panels create the openings and room layout before the exterior and interior finishes are installed.
0.14Floor W/m²K
0.16Wall W/m²K
0.14Roof W/m²K

Indicative U-values for the stated panel specification. The values for your complete build-up and design are confirmed for your project.

STEP 03

Two roof approaches. The same careful detailing.

Flat and pitched roofs give different appearances. Both need a designed structure, weatherproof covering and a route for rainwater.

Flat SIP roof with EPDM membrane

  • Structural SIP roof panels
  • EPDM single-ply membrane — Firestone in the stated specification
  • Designed roof falls, edge trims and flashings
  • Integrated rainwater goods

The specified membrane system can offer a 20-year manufacturer guarantee and an expected lifespan over 50 years. Warranty terms and service life depend on the selected product, installation and maintenance; confirm the applicable terms with your quotation.

A completed flat-roof garden building from our portfolio.

Pitched SIP roof with tiles

  • Structural SIP roof panels
  • Suitable low-pitch tile system — Marley in the stated specification
  • UPVC fascia and soffits
  • Dry verge details and integrated guttering

Low-pitch tile options can be used from 12.5° where the selected system allows. The permitted pitch depends on the specific tile, roof design and manufacturer’s requirements.

A finished pitched-roof annexe, showing the alternative roof profile.

STEP 04

What protects the outside — and gives it character.

External finishes are selected around your home, garden, preferred appearance and maintenance needs. The layers behind the finish are just as important.

External finish options

  • Western Red Cedar
  • Thermowood
  • Cedral fibre cement
  • K-Render silicone render system
  • Brick slips
  • Composite cladding
  • Feather edge timber boards

Breathable membranes and the appropriate cladding build-up help protect the structure and manage moisture. Cavities, fixings and junctions are detailed to suit the chosen system.

Explore external cladding choices ↗
Timber cladding beside the glazed entrance.
The finished external envelope of a longer garden building.

STEP 05

Turn the insulated shell into a usable room.

The interior is planned around the purpose of the building: a workspace, gym, studio or annexe with everyday living facilities.

Flooring, lighting, glazing and air conditioning in a completed room.

Room finishes and electrical installation

  • Electrical installation and agreed socket positions
  • Lighting and internal doors
  • Laminate or engineered flooring options
  • Heating, cooling and ventilation planned for the space
Kitchen cabinetry and appliances installed inside an annexe.

Annexe kitchens and shower rooms

  • Kitchen installations for granny annexes
  • Plumbing and appliance connections
  • Standard or luxury shower room options
  • Water and waste connections coordinated with the site
Kitchen options ↗ · Shower rooms ↗
A fitted garden office with shelving and a dedicated workspace.

A finished space for everyday use

We agree the interior fit-out as part of the project scope, so the finished space meets the intended use. Required electrical, plumbing and building documentation is confirmed for the work being carried out.

STEP 06

Drainage, water and power: plan them early.

A building with a kitchen or bathroom needs more than an insulated shell. The routes, levels and connection points for services affect the design and the work needed on site.

Gravity drainage

Where site levels allow, wastewater can flow by gravity to a suitable connection. Routes, gradients and approvals are assessed for the proposal.

Mini sewage pump systems

Where gravity drainage is not viable, a pump system can be assessed. The specification, discharge connection, access and ongoing care need to be planned together.

Surface-water soakaways

Modular crate systems can manage surface water where ground conditions and the drainage design allow. Suitability, capacity and the required approvals are checked for the site.

WATCH THE PROCESS

Watch our building footage.

Watch with playback controls, or expand the video for a closer view.

THE FINISHED RESULTS

From construction details to completed buildings.

Explore more examples of finished TRJ buildings from our construction portfolio.

Timber-clad garden building with full-height glazing

Timber-clad garden building with full-height glazing

Explore completed projects ↗

OUR APPROACH

A clear specification, tailored to your project.

Structural performance and long-term value

Our construction approach focuses on substantial SIP roof and floor panels, structural wall panels, professional external finishes and integrated electrics. The agreed specification makes clear what is included in your build.

We bring over 22 years of industry experience to the planning and construction process, and shape the layout, materials and finishes around each customer’s requirements.

Experienced team and ongoing support

We listen to your goals, discuss the site and explain the choices. The aim is a finished building suited to its use, with the relevant handover information and support for questions after completion.

View our services ↗
“Costi and the guys at TRJ were exceptional! They were courteous, punctual, professional and exquisite workmanship.”Futing Li, Bromley
“We’re really happy with the bar/garden office Costi has built for us. The work is of a high standard and the touching finishes are great.”MA1NE7 — Google review

Questions about the build process.

How are SIP garden rooms and granny annexes built?

The process starts with a site survey and a foundation design. The base, structural insulated panels and roof are assembled, followed by weatherproofing, external finishes, services and the agreed interior fit-out. The specification depends on the building and site.

Which foundation system is right for my garden building?

Screw piles and reinforced concrete rafts are foundation options used by TRJ. A steel frame can form the supporting base for the SIP floor, supported by the specified foundations. Ground conditions, loads, access and site levels determine the arrangement, which is confirmed after assessment.

Can a SIP garden room be used throughout the year?

A properly specified SIP building can provide comfortable year-round use. Insulation, glazing, airtightness, ventilation and suitable heating or cooling all contribute; performance depends on the complete building design.

Can you include a kitchen, shower room and utility connections?

Yes, these can be included in the agreed annexe specification. Water, power and drainage routes need to be assessed, and the scope and any required approvals confirmed before installation.

Do all garden buildings need planning permission or building control approval?

Requirements vary with the proposed use, dimensions, location and property restrictions. A self-contained residential annexe needs particular care. Check the requirements for your proposal with the relevant local authority before work starts.

Let’s discuss how your building will be made.

Send us your location, available space and intended use. We’ll talk through the site, foundation options, specification and next steps.

Discuss your project ↗