How Our SIP Garden Rooms & Granny Annexes Are Built
At TRJ Construction Ltd, we specialise in explaining how SIP garden rooms and granny annexes are built from the ground up. From engineered foundations to advanced SIP panel assemblies, this page outlines the full construction process and options for long-lasting, energy-efficient spaces.
Engineered Foundations Explained
A building is only as strong as its base. That’s why we offer two professionally specified foundation systems depending on soil conditions and building size.
Screw Pile Foundations (Low Carbon Option)
• Installed up to 1.2m deep
• Ideal for uneven or restricted access sites
• Minimal ground disturbance
• Fast installation
• Environmentally friendly alternative to large concrete slabs
Screw piles provide exceptional structural stability while reducing disruption to your garden.
Reinforced Concrete Raft Foundations
• Used for larger annexes or specific ground conditions
• Fully level structural base
• Damp proof membrane protection
• Designed to meet building control requirements
Foundation choice is based on site survey and structural assessment — not guesswork.
Unlike traditional timber frame builds, our structures are built using SIP panels for:
• Floors
• Walls
• Roofs
Each panel consists of:
• 12mm structural OSB boards
• Solid polyurethane (PUR) insulation core
• Precision joining splines
• Airtight sealing between panels
This creates a load-bearing, highly insulated and airtight shell.
Thermal Performance
Floor U-Value: 0.14 W/m²K
Wall U-Value: 0.16 W/m²K
Roof U-Value: 0.14 W/m²K
Because insulation is built into the panel itself, there are no poorly fitted gaps and significantly reduced cold bridging.
The result: a building designed for true year-round use.
Roof System Options
We offer both flat and pitched roof systems depending on design preference.
Flat Roof SIP Construction
• Structural SIP roof panels
• Firestone EPDM single-ply membrane
• 20-year manufacturer guarantee
• Expected lifespan over 50 years
• Integrated rainwater goods
EPDM provides 100% waterproof protection and long-term reliability.
Pitched Roof SIP Construction
• SIP structural roof panels
• Marley low-pitch tiles (from 12.5°)
• UPVC fascia and soffits
• Dry verge system
• Integrated guttering
This option allows traditional aesthetics with modern performance.
External Finish Types & Why They Matter
Your building exterior can be tailored to suit your home and personal taste.
Options include:
• Western Red Cedar
• Thermowood
• Cedral fibre cement
• K-Render silicone render system
• Brick slips
• Composite cladding
• Feather edge timber boards
All systems are installed over breathable membranes to protect the building envelope and maintain moisture control.
This ensures both visual appeal and long-term durability.
Interior Fit-Out & Functional Spaces
Once the SIP shell is complete, we integrate:
• Electrical installation
• Plumbing (for annexes)
• Laminate or engineered flooring options
• Internal doors
• Kitchen installations (for granny annexes)
• Shower rooms (standard or luxury)
All installations are carried out to current building standards where required.
Our aim is to deliver a fully functional, ready-to-use living space — not just an empty structure.
Drainage, Utilities & Compliance
For annexes and buildings requiring facilities, we provide:
Gravity Drainage Systems
Designed and installed in line with British standards and building control approval.
Mini Sewage Pump Systems
Used where gravity drainage is not viable due to site levels.
Soakaway Systems
Installed using modular crate systems to manage surface water efficiently.
Every drainage solution is tailored to the site layout and compliance requirements.
What Makes TRJ’s Method Different
Many companies reduce panel thickness to reduce price.
At TRJ Construction, we focus on structural performance and long-term value.
We use:
• 150mm SIP panels for roof and base(for low level buildings under 2.5m high )
• 125mm structural insulated wall panels(for low level buildings under 2.5m high )
• Professional K-Render finishes
• Fully integrated electrics as standard
This approach delivers stronger buildings, better insulation and improved durability — without cutting corners.
Passion and Expertise in Every Project
With over 22 years of experience in the industry, we are truly passionate about what we do, and it shows in every project we take on. Our deep knowledge of the field ensures that no challenge is too big or too small. We put our full dedication and energy into delivering exceptional results for every client, no matter the scope of the project.
Tailored Solutions for Every Customer
At TRJ Construction Ltd, we believe that every customer is unique. That’s why we create customized plans designed to meet your specific needs. Whether you're looking for a small project or a large-scale effort, we take the time to understand your goals, listen to your ideas, and develop a plan that’s perfect for you.
Award Winning
We’ve built more buildings than we can count but we don’t let it go to our heads. We dedicate ourselves to every project.
Expert Team
Your project will be handled by experts every time. We make sure you have the most experienced professionals working for you.
Quality Guaranteed
You’ll find the support you need to make sure that things runs smoothly. We’re here to help you with any questions.
Premium Garden Rooms
Premium Granny Annexe N London |SIP panels installers N London |Garden Rooms Essex
“Costi and the guys at TRJ were exceptional! They were courteous, punctual, professional and exquisite workmanship. I couldn't ask for anymore and I'm confident anyone else who is considering using his service will agree. Highly recommend.”
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. Costi was very friendly and knowledgeable and clearly takes pride in his work. I thoroughly recommend using him and his company. Thanks again.”
MA1NE7, google review
