Bespoke SIP Garden Room with Shower Room in Mill Hill
A fully insulated garden room designed for work, fitness and everyday use
This bespoke garden room in Mill Hill, North London was designed as a high-quality multi-use space that combines a bright main room with a fully fitted shower room and provision for a small kitchenette. Built using a SIP panel structure, the project was created to deliver a comfortable, practical building for year-round use, with the flexibility to function as a home office, gym, studio or private garden retreat. The order form shows an external area of 28.5 sqm, with the layout arranged to include an open main room, shower room and storage for day-to-day living.
This project was not straightforward from a groundworks point of view. Because of a nearby willow tree, the foundation design had to go much deeper than standard, and the screw pile foundations were ultimately extended to around 3.2 metres deep to achieve a safe and suitable solution for the site. TRJ Construction also handled the planning permission for the client, helping move the project from design through approvals and into construction.
Project Overview
This garden room was designed for a client in Shakespeare Road, Mill Hill, as a permanent, properly built outdoor room rather than a basic summer structure. The visuals and layout show a clean modern building with a front glazed elevation, white rendered exterior and a mono-pitch roof profile. The internal layout was designed to support multiple uses, including workspace, gym use and day-to-day convenience, with a dedicated shower room and allowance for a small kitchenette.
What makes this project especially important is that it combined design, approvals and technical groundworks. TRJ did not just build the structure — we also took care of the planning permission, and adapted the foundation strategy to suit difficult site conditions caused by the willow tree nearby.
Size and Layout
The order form shows this building at 28.5 sqm external area, with the drawing laid out as a versatile internal space including:
- a large main open-plan room
- a standard shower room
- allowance for a small kitchenette
- full-height front glazing for natural light

Planning Permission and Project Support
A major advantage on this project was that TRJ Construction handled the planning permission for the client. For many customers, this is one of the most stressful parts of creating a larger garden building, especially when the building includes plumbing, drainage, a shower room and a more substantial structure.
By managing the planning side as part of the overall process, we helped the client move forward with a clearer route from concept to approval and then into construction. This kind of joined-up approach is a major benefit when delivering more advanced SIP garden rooms.
A Challenging Foundation Solution
This project also required a more technical foundation approach than usual. The order form notes that the standard foundation was based on screw piles at 1250mm deep, with deeper screw pile extensions agreed based on geological and soil testing and building control requirements.
In practice, because of the willow tree close to the build area, the foundations had to be taken much deeper, reaching around 3.2m in order to provide the right support for the building and satisfy the site conditions. That is exactly the kind of real-world challenge that can affect a project, and it is why proper assessment, engineering judgement and the right foundation system matter.
This is also a good example of why SIP buildings and screw pile systems work so well together on more demanding sites: they allow a strong, engineered structure to be delivered while adapting the foundation depth to the actual ground conditions.
Built with SIP Panels
The structure of the building uses SIP panels for the base, walls and roof, giving the client a much higher standard of build than a typical timber garden room. The order form and specification pages describe SIPs as a high-performance building system offering thermal efficiency, structural strength and faster installation than many traditional methods.
That means this garden room is designed to feel like a proper insulated room, not just an outdoor building used only in mild weather.
Roof, Structure and Engineering
The specification also includes:
- metal beams above the main opening and inside the roof
- structure built in line with engineer design
- EPDM rubber roofing
- external membrane, battens and wall ventilation system
These details show this was designed as a proper engineered garden building, not a light prefabricated shell.
Interior Finish
Inside, the project was designed to be fully finished and ready to use. The specification includes:
- internal plasterboard and skim
- white Dulux paint
- Howdens laminate flooring
- white skirting
- Howdens 4-panel shaker internal doors
This gives the interior a clean, modern and practical finish suited to both work and leisure use.
External Finish and Design
The design of this project is simple, modern and clean. The order form specifies:
- cement particle board external cladding
- upgraded white silicone K-render
- EPDM roof membrane with black trim
- black round gutter system
- front glazing with French doors and tall side windows
Doors and Windows
The specification includes:
- 1 set of French doors, approximately 1600mm x 2100mm high
- 2 fixed-frame side windows, approximately 800mm x 2100mm high
- 1 casement window with obscure glass for the toilet area
- all in white, using toughened, argon-filled double glazing
This gives the main room a bright, open feel, while the obscure glazed bathroom window provides privacy where needed.
Electrical and Heating Package
The electrical setup on this project is stronger than what many standard garden rooms include. The order form specifies:
- independent fuse board
- RCBO and surge protection
- 6 double sockets
- 8 internal downlights
- 3 external downlights
- LED bathroom light
- 1 Haverland 1kW wall radiator
- fire alarm included
- connection to water and electric included
This makes the building practical from the start and suitable for more demanding day-to-day use.
A key feature of this project is that it was designed with proper facilities rather than just an empty main room. The order form includes a standard shower room, based on an agreed specification, with gravity discharge. It also includes an allowance for small kitchenette units and sink, with fitting included.
The shower room specification page also shows that the standard package included a shower tray, sliding shower door, basin tap and waste, vanity unit with basin and toilet, extractor fan, waterproof light, towel radiator, electric shower and waterproof laminate flooring.
That makes this building much more flexible than a standard office-only garden room.
Why This Project Stands Out ?
This Mill Hill project stands out because it is more than just a nice-looking garden room. It combines:
- SIP construction for year-round comfort
- a proper shower room
- kitchenette provision
- rendered contemporary exterior
- engineered structure and roof support
- full electrics and heating
- planning support handled by TRJ
- a specialist deep foundation solution due to the willow tree
It is a strong example of how a bespoke SIP garden room can be tailored to a difficult site and still deliver a clean, practical and attractive finished building. The file shows both the technical specification and the intended layout for multi-use everyday living.
Thinking About a Similar Project?
At TRJ Construction, we design and build bespoke SIP garden rooms that go far beyond a basic garden building. Whether you need a garden office, gym room, studio or a fully serviced outdoor room with shower and kitchenette, we can help with the full process — from design and planning through to foundations, structure and final fit-out.
Need a bespoke SIP garden room with planning support?
Speak to TRJ Construction about your project and we can help you design the right layout, finishes and foundation solution for your site.
Exemplary company. 🥇
Unbeatable in every way
Having had so many bad building experiences, finding Costi from TRJ Construction was a miracle. The space given here would not be large enough to give all the positive feedback other than to say it was a uniquely positive experience. I am happy to show our new flatlet off to any prospective customer. It’s fantastic and finished ahead of time."
Exemplary company. 🥇
Unbeatable in every way
Having had so many bad building experiences, finding Costi from TRJ Construction was a miracle. The space given here would not be large enough to give all the positive feedback other than to say it was a uniquely positive experience. I am happy to show our new flatlet off to any prospective customer. It’s fantastic and finished ahead of time.
Interested in your own bespoke granny annex ? Contact us today to start your project with a team that delivers excellence, speed, and custom solutions tailored to your needs.
