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Rear extension · Ground floor

Building a rear extension

More living space by extending your existing home towards the rear

Do you want to enlarge your living room or kitchen with an extension at the rear of your home? A rear extension can considerably improve the existing living space. A closed kitchen can become a spacious open-plan kitchen, a small living room can be extended towards the garden, and a wide sliding door creates more light and a direct connection with the outside.

Structurally, however, a rear extension is more than just a new volume behind the house. The existing load-bearing rear wall is often removed in whole or in part. The load from the upper floor and the wall above must then be carried by a steel beam or portal frame. At the same time, the new floor, foundation, walls and roof structure must be designed safely.

ROMB handles the architectural and structural design of your rear extension, from foundation and floor to opening the wall, steelwork and roof.

Discuss your extension

What is a rear extension?

In practice, a rear extension (in Dutch: uitbouw) usually means that an existing room of the house is enlarged. For example:

The existing rear wall is then often largely opened up, so that the existing home and the new extension form a single space. That distinguishes a typical rear extension from a separate annex (aanbouw), where a separate utility room, study or other new room is built against the existing home. Structurally, both projects can contain comparable elements.

Which structures are needed for an extension?

A rear extension consists of several structural elements that together form a single load-bearing structure. Think of:

In addition, the new structure must connect to the existing home. The key question is therefore not only: “Which beam do I need?” but: “How are all the loads of the new and existing structure carried safely down to the foundation?”

Opening up the existing rear wall

In many extensions, opening up the existing rear wall is the most important structural element. The rear wall can carry loads from:

When this wall is removed in whole or in part, these loads must be taken over by a new structure. Depending on the width of the opening and the existing situation, the choice may fall on:

We calculate which solution is technically and practically suitable.

Removing a load-bearing wall

Steel beam or a complete portal frame?

For a smaller opening, a steel beam can sometimes be supported directly on sufficiently strong masonry. When almost the entire rear wall is removed, there is often insufficient wall left to support the beam safely. In that case a steel portal frame may be needed. A portal frame usually consists of:

We calculate, among other things:

Portal frame calculation

Not just the steel, but the full load path

A structural calculation does not stop at the steel beam. The load must ultimately be carried down to the ground:

upper floor / wall above→ steel beam→ columns or supports→ floor / foundation beam→ foundation→ subsoil

It must therefore be investigated where the new columns or supports end up. A heavy steel beam can be strong enough in itself, while the structure below it has insufficient capacity. Additional measures may then be needed, such as:

Foundation of the extension

The foundation forms the basis of the new extension. Which foundation is needed depends, among other things, on:

Possible solutions include:

The right foundation method must always be determined project-specifically.

Extension foundation

Extension on piles

In areas with soft soil, for example parts of Rotterdam and other parts of the western Netherlands, a piled foundation may be necessary. The extension can then be carried by foundation beams that rest on piles. Among other things, the following are determined:

The number and type of piles can only be responsibly determined after the load, soil data and foundation set-up have been assessed.

Connection to the existing foundation

A frequently asked question is whether the new extension can simply be connected to the existing foundation. That is not always self-evident. The existing home may, for example:

Moreover, the new extension can respond differently to settlement than the existing home. It must therefore be carefully determined how the two structures connect.

Ground floor of the extension

The new floor must also be designed as a structural element. Frequently used solutions are:

Which floor is suitable depends, among other things, on:

For an extension above a crawl space, an EPS composite floor is a frequently used solution.

EPS / beam-and-block floor

Floor height and connection to the living room

An important practical point of attention is the floor level. The new floor often consists of several layers:

The total thickness of these must be coordinated in advance with the existing floor. Otherwise an unwanted difference in level can arise after construction between the existing home and the new extension. We therefore include the structural floor height in the technical design.

Roof structure of the extension

A rear extension is often given a flat roof. The load-bearing structure can consist, for example, of:

In the calculation we take account of relevant loads from, among other things:

If you are considering a green roof, roof terrace or other heavier use function, this must be included in the design in advance.

Large sliding door on the garden side

Many extensions are given a wide sliding door, folding wall or large glazed section. This creates an open connection with the garden, but structurally less solid wall remains available. A beam may be needed above the door set to carry the roof. The remaining side sections of the wall also play a role in:

The rear of the extension is therefore regarded as part of the overall structural system.

Stability of the extension

An extension must not only be able to carry vertical loads. Wind also causes horizontal forces. These forces must be carried safely down to the foundation via the roof and walls. In an extension with traditional masonry side walls, stability can often be provided through these walls. In designs with:

additional structural stability may be needed. For example through:

Temporary support during the wall opening

Before the existing rear wall is removed, the structure above must be safely supported on a temporary basis. Construction props and temporary needle beams can be used for this, for example. What matters is not only how many props are needed. It must also be assessed:

In more complex situations a separate propping plan may be needed.

Propping plan

Structural calculation for the extension

Depending on the project, we can calculate, among other things:

Where necessary, we also check relevant parts of the existing home.

Structural drawings for the contractor

The structural solution is recorded on clear technical drawings. These can show, for example:

This way the contractor knows not only which profile to order, but also how the complete structure must be built.

Architectural drawings for the extension

In addition to structural drawings, architectural drawings may be needed for the design, the construction and possibly the permit. Think of:

ROMB can combine the architectural and structural drawing work within a single commission. This means the structure can already be coordinated during the design.

Is a rear extension permit-free?

It can be, but not every extension is automatically permit-free. Under the Dutch Environment Act (Omgevingswet) a distinction is made between the technical part of building and the spatial part. An accessory structure can, under the Bbl, be permit-free for the technical part when conditions such as the following are met:

A structural change that is part of such a permit-free extension, such as an opening in the existing rear wall, can also be permit-free for the technical part. For the spatial part, it must additionally be checked whether the plan fits within the rules of the municipal environment plan. Among other things, the following may be relevant:

The permit requirement must therefore always be checked for the specific address and design via the Omgevingsloket. Even when no environmental permit is required, the extension must comply with the applicable technical rules of the Bbl.

Have the feasibility assessed

Why structural calculations even for permit-free building?

Permit-free does not mean structure-free. A contractor still needs to know:

Moreover, the Bbl continues to set technical requirements for the structure. Structural calculations are therefore not only intended for the municipality, but above all for a safe and verifiable construction.

Architectural and structural connection

In an extension, the existing home and the new structure literally come together. We therefore pay attention to, among other things:

These details are important to prevent problems during and after construction.

Drainage and services

An extension is regularly combined with a new kitchen. As a result, technical services may also be changed. Think of:

Pipes and openings must be coordinated with:

Running a pipe through a structural beam afterwards is not simply possible. We therefore advise determining important pipe routes early in the design.

Daylight and ventilation

A deep extension can affect the daylight in the existing part of the home. The following design choices therefore play a role, among others:

Ventilation provisions must also be coordinated with the new situation. When building-physics calculations are needed for the project, these can be provided additionally.

First make drawings, or first choose a contractor?

A contractor can give a rough price indication based on your wishes. For a properly comparable and technically reliable quotation, however, it is wise to first be clear about what actually needs to be built. With architectural and structural drawings, contractors know, for example:

This makes quotations more comparable with one another and reduces the chance of unexpected additional work during construction.

Our method of working

  1. Assessing your plans. You send us the project address, available drawings, photos and the desired dimensions of the extension.
  2. Investigating the existing situation. We look at relevant data about the load-bearing structure, floor, rear wall, foundation and earlier alterations.
  3. Architectural design. Where part of the commission, we work out the new floor plan, elevations and cross-sections.
  4. Structural design. We determine the principle for foundation, floor, wall opening, steelwork and roof.
  5. Calculations. The relevant structural parts are calculated.
  6. Structural drawings. We produce a clear technical set for construction and, if needed, assessment by the municipality.
  7. Coordination and permit. Where agreed, we coordinate the structure with the contractor, architect or municipality and support the required application.

One coherent design

In an extension, the individual structural parts are strongly connected. A wider wall opening, for example, often means:

wider opening→ heavier beam→ greater column force→ higher foundation load

A different floor or larger roof can also affect the foundation. We therefore do not only calculate isolated parts, but look at the complete load path of the extension. That results in a structure that is not only theoretically strong enough, but can also be built in practice.

For general building advice or projects outside our structural specialism, we work together with Aboss Bouwadvies.

Frequently asked questions about a rear extension

What is the difference between an annex and a rear extension?

In everyday use the terms are used interchangeably. A rear extension usually means that an existing room, for example the living room or kitchen, is enlarged. An annex can also form a new, separate room.

Can I remove the entire rear wall?

That is often technically possible, but a calculated steel structure is usually needed. For large openings a steel portal frame may be necessary.

Do I always need a new foundation?

A suitable foundation solution is needed for the new part. Which foundation is suitable depends on soil, load and the existing situation.

Can my extension be connected to the existing foundation?

That must be assessed per project. The existing foundation, new load and settlement behaviour all play a role.

Which floor is suitable for an extension?

An EPS composite floor or an insulated concrete floor, among others, can be applied. The choice depends on the foundation, span and desired floor build-up.

Is a permit always needed for an extension?

No. Under conditions, an extension can be permit-free for the technical and/or spatial part. Both parts must be checked separately for the project.

Is a structural calculation needed if I build permit-free?

A permit-free extension must still be structurally safe and comply with the Bbl. For load-bearing parts, calculations are therefore often needed for a responsible construction.

How do I know which steel profile is needed?

That cannot be determined solely on the basis of the width of the opening. Floor loads, walls above, supports, columns and the foundation must also be assessed.

Can you also calculate foundation piles?

When a piled foundation is part of the design, we can include the loads and structural foundation set-up. Any required soil investigations (CPTs) or specialist soil data are taken into account.

Can you make both the building drawings and structural calculations?

Yes. We can provide both the architectural and structural part of an extension.

Can you use an existing drawing from my architect?

Yes. We can base the structural design on existing architectural drawings and coordinate with the architect where necessary.

Do you also work with my contractor?

Yes. Technical questions about the structure can be coordinated with the contractor, steel fabricator or other parties involved where necessary.

Would you like to build a rear extension?

Do you want to enlarge your living room or kitchen and know which structure, foundation and steel solution are needed for it?

Send us the project address, existing drawings, photos of the rear, the desired dimensions of the extension and any design drawings already made. We can then assess which architectural drawings, structural calculations and technical documents are needed for your project.

Have your extension assessed