September 2025 – Timber Frame Extension Details

This month we’ve added a new series of details focused on timber frame house extensions, offering practical solutions for a variety of boundary, floor, and wall junction conditions. These details have been developed to address common scenarios faced when designing extensions to existing buildings—especially where retaining or upgrading existing construction needs to interface with new build elements.

These details are featured in the new book House Extension Details by Aida Rodriguez Vega and Emma Walshaw.

Foundation and Floor Options for Timber Frame Extensions

The new set includes multiple options for ground floor construction, covering both beam and block suspended floors and solid floor slabs. You’ll find variations including:

• Trench fill foundations with beam and block floors
• Trench fill with RC slab
• Reinforced concrete raft foundations

Each detail addresses insulation continuity, moisture protection, and includes structural considerations for packers, vents, and upstands. The goal is to minimise thermal bridging while ensuring robust and adaptable construction.

Party Wall and Boundary Conditions

Designing extensions that connect to or near party walls introduces additional complexity. The new EPTW series addresses this with a clear and well-annotated set of plan and section details showing:

• Walls offset from the party line to allow for parapet overhang
• Details aligned with the existing face of masonry
• Corner conditions with options for brick and timber cladding returns
• Junctions with new cavity masonry party walls

We’ve also included thermal timber wall options for situations where the party wall cannot be structurally relied on, such as cantilevered RC slab options.

Refurbishment Interface Details

Finally, the EFTF series explores how to tie new floors into existing buildings. These are often overlooked junctions that require thoughtful detailing, especially when connecting to existing suspended timber floors. Options shown include:

• New suspended timber floor to existing wall
• Beam and block floor with new timber structural wall
• RC slab floor abutting existing suspended floor, with ventilated detail options

Use and Adaptation

As with all our details, these are intended as a guide to good practice and should be reviewed and adapted by your structural engineer and design team. Each drawing includes practical notes on insulation, moisture protection, and sequencing.

We hope this new set supports your design process when working with extensions to existing homes, particularly in tight boundary conditions or where continuity with existing structures is key.

Explore the full set below.

New Details