Trees and Clay Soil

 

Trees can have a significant effect on foundation design, particularly where the property is located on shrinkable clay soil.

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The type of tree, its mature height and its distance from the proposed foundations may all need to be considered.

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This can sometimes result in deeper foundations or additional precautions.

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It is sensible to identify nearby trees at an early stage rather than discover the issue after construction has begun.

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Drainage and Structural Design

 

Drainage can also affect the structural design.

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An extension may be close to:

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• private drainage runs;

• public sewers;

• inspection chambers;

• manholes; or

• rainwater drainage.

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The position of the foundations may need to be adjusted, and some drains or inspection chambers may need to be relocated.

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Where work is close to or over a public sewer, a separate build-over agreement may be required from the relevant water company.

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Drainage and foundation design should be considered together.

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Roof Structures and Rooflights

 

The roof is another important part of the structural design.

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Many homeowners want rooflights, lanterns or large areas of glazing to bring more daylight into an extension.

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These features can create an attractive interior, but the roof still needs to be supported properly.

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The structural design may need to consider:

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• roof joists;

• rafters;

• trimming around rooflights;

• supports for roof lanterns;

• flat-roof spans;

• pitched-roof construction;

• wind loads; and

• the relationship between the roof and any large openings below.

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A visually simple roof can still require careful technical design.

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Side-Return and Wraparound Extensions

 

Side-return and wraparound extensions can involve more complex structural decisions than a straightforward rear extension.

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The design may require the removal of sections of the original rear wall and side wall to create a wider kitchen and family space.

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Depending on the layout, structural support may be needed in more than one direction.

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Victorian and Edwardian properties can also have varied construction methods, uneven floor levels and older rear additions that need to be considered carefully.

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A coordinated design is particularly important where large openings, rooflights and side-return glazing are combined.

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Double-Storey Extensions

 

A double-storey extension introduces additional loads because the new foundations and walls must support both floors and the roof above.

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The structural engineer may need to consider:

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• the increased foundation requirements;

• load-bearing walls;

• floor joists;

• roof construction;

• steel beams;

• openings between the original house and the extension;

• lateral stability; and

• the relationship with the existing structure.

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The architectural layout and structural design should be developed together.

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Extensions Combined With Loft Conversions

 

Where a homeowner is planning an extension and a loft conversion at the same time, both projects should be considered as part of one coordinated design.

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The loft conversion may introduce new steelwork and additional loads at upper-floor level.

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The extension may involve removing walls or changing the structure below.

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Designing the two projects separately can create avoidable problems.

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A coordinated approach helps ensure that the structure works properly from the foundations through to the roof.

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Internal Alterations Can Also Need Structural Design

 

Structural calculations may be needed even where the proposed work is largely internal.

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Common examples include:

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• removing a load-bearing wall;

• creating a wider opening between two rooms;

• removing a chimney breast;

• altering a staircase;

• opening the rear of the house;

• adding large doors or glazing; and

• changing the roof structure.

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A wall should not be assumed to be non-load-bearing simply because it appears thin or inconveniently positioned.

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It is better to confirm the position before building work begins.

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Structural Calculations and Building Control

 

Structural calculations will normally be submitted as part of the Building Regulations process where structural alterations are proposed.

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The Building Control body will review the information alongside the Building Regulations drawings.

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The structural calculations may set out:

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• the loads acting on the structure;

• the proposed beam sizes;

• supporting columns;

• padstones;

• connection details;

• foundation requirements;

• structural sketches; and

• relevant construction notes.

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The final structural details should be available before the builder orders steelwork or carries out major alterations.

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Why Structural Design Should Be Completed Before Builders Quote

 

Builders can provide an early budget estimate from planning drawings.

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However, a reliable quotation is much easier to obtain once the structural calculations and Building Regulations drawings have been prepared.

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Without this information, contractors may be making different assumptions.

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One builder may allow for a simple beam.

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Another may anticipate a goalpost frame, supporting columns and additional foundations.

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A third may leave important structural items out of the quotation entirely.

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This makes it difficult to compare prices properly.

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Detailed structural information helps reduce uncertainty and makes unexpected extras less likely during construction.

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Can the Builder Arrange the Structural Engineer?

 

Some builders have engineers they work with regularly.

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This can be suitable for a relatively simple project.

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However, arranging the structural design before appointing the builder can give the homeowner more control and make quotations easier to compare.

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The builder is then pricing the same set of drawings and calculations as the other contractors.

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For a more complex extension, it is usually better to resolve the main structural design before agreeing the construction contract.

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Temporary Support During Construction

 

Structural design is not only about the finished extension.

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Walls and other structural elements may need to be removed carefully while the house is temporarily supported.

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The builder is responsible for planning a safe construction sequence and providing suitable temporary support.

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This may include propping the structure before beams or columns are installed.

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Structural alterations should never be treated casually, particularly where part of the existing house remains occupied during the work.

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Structural Design Is Part of a Successful Extension

 

A well-designed extension needs more than an attractive layout and a set of planning drawings.

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It needs a clear understanding of how the loads will be supported, how the existing house will be altered and how the structure will be constructed safely.

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Resolving the structural design early helps:

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• reduce uncertainty;

• improve builder quotations;

• support the Building Regulations application;

• avoid preventable delays;

• reduce unexpected costs; and

• produce a better finished extension.

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Tell Us About Your Extension

 

Planning a rear extension, side-return extension, wraparound extension, double-storey extension or a larger renovation project?

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Tell us a little about your property and what you are hoping to achieve. Where appropriate, we can help you identify the professional support you may need, from architectural design and planning advice through to Building Regulations drawings, structural engineering and suitable extension contractors.

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