Timber extension planning guide

Investigate before fixing the form

Archive drawings are clues, not proof of what was built. Measured survey and targeted investigation by competent parties can identify build-ups, structure, services, moisture and possible hazardous materials. Boundaries, levels, drainage and delivery access also constrain the realistic volume.

Timber’s lower self-weight and prefabrication can be advantageous. The limitation is an existing building that may have settled, moved or deviated from drawings. Precisely made elements need a tolerance and final-measurement plan.

Reconcile records with measured survey
Mark load-bearing and stability elements
Investigate services, drainage and hazards
Survey delivery and assembly access

Decision: separate or structurally connect?

An independent frame with a movement joint behaves differently from a load-sharing connection. Structural and envelope designers must coordinate movement, weathering and airtightness.

Resolve four critical interfaces

At foundation level, new ground and old footings can settle differently. Ground information and existing-foundation investigation inform the solution. A new opening changes load paths and stability; temporary works and sequence require professional design, not homeowner engineering.

At roof, vertical joint and threshold, details must shed water outward, continue the air barrier and permit appropriate drying. Movement must not overload seals. Thermal bridging, fire and acoustics need a joined-up detail. The timber design should define construction and architectural intent without assuming a particular manufacturing location or material origin.

Foundation and differential movement
Opening, support and stability
Roof drainage and joint cover
Threshold, air barrier and thermal bridge

Align approvals, sequence and ownership

An extension can alter setbacks, use, fire strategy, drainage and other controlled matters. Requirements vary by jurisdiction; the local designer and authority must confirm them. German projects specifically require checking the applicable state building rules, local plan and procedure; this is not legal advice.

Use an interface matrix: who supplies survey information, designs demolition and temporary support, releases dimensions, seals each joint and inspects concealed work? Add weather and occupancy plans. Handover should include photographs of closed details and updated drawings.

Confirm the local approval path
Agree demolition and assembly sequence
Plan weather protection and occupancy
Schedule hold points before closing

Hypothetical kitchen extension

A hypothetical single-storey timber addition enlarges a kitchen. Survey finds a stepped plinth and investigation reveals an undocumented service. The team relocates the opening, designs a movement joint and coordinates roof water and airtightness before fabrication. This is a planning scenario, not a customer case.

Pitfalls to avoid

Treating archive drawings as exact as-built information
Designing roof and threshold joints on site
Ordering panels before approvals and temporary support are resolved

Frequently asked questions

Is timber automatically easier to approve?

No. Approval follows the proposal and local rules, not the material alone.

Can existing foundations carry it?

Only project-specific investigation and design can answer that.

Is a movement joint always required?

The structural and envelope strategy determines whether and how movement is accommodated.

Can occupants remain during work?

Possibly, with a project plan for safety, dust, weather, utilities and escape.

Related guides

Sources

These sources explain general planning principles, mainly in the German context. They do not confirm project scope, permission, funding or technical suitability. Local requirements need separate professional assessment.

Plan a timber extension around the existing building