Bespoke 40m² Granny Annexe in Waltham Cross, Hertfordshire

The success of this annexe came from planning the rooms around how the space would actually be used. Instead of filling the footprint with corridors, the design gives most of the area to the main living zone while still providing the separate facilities needed for day-to-day independence.

The kitchenette sits within easy reach of the living space, while the private shower room and additional room are grouped efficiently to keep the plan simple and practical. That arrangement allows the extra room to work as a bedroom, study or quiet private space depending on the occupier’s needs.

Because the structure, openings and services were considered together from the outset, the build could progress in a logical sequence from foundation and SIP shell through to electrical work, plumbing, finishes and climate control.

This is the kind of compact annexe where careful planning matters more than simply adding floor area: every part of the 8m × 5m envelope has a clear purpose.

This building was specified as a genuine annexe rather than an upgraded summerhouse. The project includes the essential infrastructure needed for regular occupation, with dedicated electrical protection, water and drainage, heating, cooling, a kitchenette and a private shower room.

The construction specification was coordinated with Building Control considerations, including the foundation solution, structural shell and service routes. That early coordination reduces the risk of having to alter finished work later in the project.

The finished specification combines structural performance with practical details that matter in everyday use: accessible power, good lighting, ventilation to the shower area, durable flooring and independent temperature control.

What was built into this project:

• Extended screw-pile foundation system
• Structural insulated panel floor, wall and roof shell
• EPDM flat-roof covering with black rainwater goods
• Black vertical composite feature cladding to the front
• Rendered side and rear elevations on fire-retardant boards
• Fitted kitchenette and dedicated shower room
• Independent consumer unit with RCBO and surge protection
• LG 2.5kW inverter Wi-Fi heating and cooling
• Internal and external lighting, sockets, water and drainage connections

The annexe is supported on a screw-pile foundation system. For this project, the specified pile depth was increased beyond TRJ’s standard starting point, with the final requirement dependent on the soil conditions and engineering information agreed for the site.

Using screw piles keeps the structural support concentrated exactly where it is required and avoids the scale of excavation normally associated with a full concrete foundation. That is particularly useful in an established rear garden where access and disturbance need to be controlled.

Once the piles are installed and levelled, they provide a precise base for the structural platform and SIP floor, allowing the insulated shell to be assembled on a stable, accurately set foundation.

The floor, external walls and roof were specified as structural insulated panels, so the principal envelope of the annexe is formed from one coordinated construction system rather than separate framing and insulation stages.

SIPs combine structural skins with a rigid insulated core. On a compact annexe, that approach helps create a consistent thermal envelope while also allowing the shell to be erected efficiently once the base is ready.

For a building intended to be occupied in winter as well as summer, the continuity of the insulated envelope is particularly important. It works together with the heating and cooling system to make the internal temperature easier to manage across the seasons.

The exterior uses contrasting finishes to give the annexe a stronger architectural identity. The front elevation is treated as the feature face of the building, while the less prominent elevations use a rendered finish for a clean and durable appearance.

Black vertical composite cladding defines the front of the annexe. The sides and rear are finished on fire-retardant boards with anthracite-grey render, creating a simple palette that suits the black windows, entrance door and rainwater goods.

The flat roof is finished with EPDM membrane, keeping the roofline visually restrained and allowing the external design to remain clean and contemporary.

External specification at a glance:

• Black vertical composite cladding to the principal elevation
• Anthracite rendered finish to the side and rear elevations
• Black uPVC entrance door and windows with trickle ventilation
• EPDM membrane roof with a simple flat-roof profile
• Ventilated external wall build-up and black rainwater system

A self-contained annexe only works when the services are planned as carefully as the structure. This project therefore integrates the kitchenette, shower room, electrical installation, water supply, drainage and heating and cooling within the main build programme.

The electrical installation includes a separate consumer unit with RCBO circuit protection and surge protection, together with multiple socket outlets, internal lighting and external lights. Dedicated supplies are provided for the fitted equipment and climate-control system.

The shower room specification includes mechanical extraction, an electric towel radiator, waterproof finishes and an electric shower. An LG 2.5kW inverter Wi-Fi air-conditioning unit provides both heating and cooling for improved comfort throughout the year.

Before the main structure is treated as a finished design, the site has to work technically. On this project, the foundation arrangement, soil conditions, drainage route and service connections were all part of the construction planning rather than being left until the fit-out stage.

The project specification included a soil test to support the screw-pile engineering and Building Control information. Drainage was designed around a gravity system where site levels allowed, with provision to review the solution if conditions on site required a different approach.

Coordinating these items early gives the project a clearer route from groundworks to handover and helps keep structural work, plumbing and electrical installation aligned with the same final layout.

Once the insulated structure and services were in place, the focus moved to creating a finished interior that feels residential rather than temporary. The specification includes plasterboard, skimmed walls, white decoration, laminate flooring and matching skirting throughout the main accommodation.

The internal layout is supported by practical lighting, power points, a dedicated shower room and a usable kitchenette, while the separate room adds flexibility for sleeping, working or quieter private use.

The completed specification shows what can be achieved within a relatively modest garden footprint when the structure, services and room planning are treated as one project from the beginning.