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Extension

Building an extension

From first design to structure and permit

Would you like to enlarge your home with an extension? An extension can create considerably more living space and completely change a home's layout — think of a larger kitchen, a spacious kitchen-diner, extra seating, a home office or a direct connection to the garden.

A well-designed extension, however, does not begin with the build but with careful preparation. The foundation, floor, façade opening, steel structure, roof structure, insulation and connection to the existing home must be designed as one whole.

ROMB takes care of the technical preparation of your extension: from building drawings and structural calculations to foundation, steel structure and permit guidance.

Discuss your extension

What is involved in building an extension?

At first glance an extension seems a relatively simple enlargement of the home. Structurally and constructionally, however, various parts come together:

A change to one part can affect another. A wide opening in the existing rear façade may, for example, require a heavier steel structure. This creates larger point loads, so the foundation must also be reassessed. That is why we prefer to treat an extension as one coherent architectural and structural design.

Extension or add-on: what is the difference?

In practice the terms are often used interchangeably. An add-on usually enlarges an existing room, for example by extending the living room or kitchen further back. An extension may be a new room built against the home, such as a utility room, office or extra living space.

For the technical development this terminology usually makes little difference. In both cases the foundation, floor, façade opening and connection to the existing home must be correctly designed.

First examine the existing home

Before an extension can be worked out, it must be clear how the existing home was built. Where possible, we assess:

In older homes the actual situation may differ from the original drawings. An additional survey or on-site check may therefore be necessary.

Design and building drawings for the extension

A good building drawing forms the basis for both the structural calculation and the execution. Depending on the project we can produce:

These drawings can then be used for: the permit application, the structural calculation, the contractor's quotation and the execution on site.

Building drawings extension

Foundation of the extension

An extension needs a foundation that suits:

Depending on the location and existing situation, options include, for example:

In areas with soft soil layers, such as large parts of the western Netherlands including the Rotterdam region, a pile foundation may be necessary. Which solution is suitable must be determined per project. We look not only at the new extension, but also at possible differences between the existing and new foundation.

Foundation calculation extension

Preventing differential settlement

An existing home and a new extension can respond differently to loading and soil conditions. When both structures settle differently, this can cause, among other things:

The foundation concept must therefore be chosen carefully. Sometimes the existing and new foundation are structurally coupled. In other situations a controlled separation or movement joint is preferable. The right solution depends on the existing foundation and the local soil conditions.

Ground floor of the extension

Several systems are possible for the floor of an extension. Commonly used are:

The choice depends, among other things, on: foundation, span, available structural height, crawl space, insulation requirements, pipe routing, underfloor heating and connection to the existing floor.

In a combination floor, prefab concrete beams are combined with insulating EPS infill blocks and a concrete topping. This system can be very suitable for an extension over a crawl space.

Beam-and-block floor extension

Connection to the existing floor

A point regularly underestimated in the design phase is the final floor level. The structural floor is only one part of the total floor build-up. Above it may come, for example:

The level that must ultimately connect to the existing living room or kitchen should therefore be established in advance. This prevents an unwanted difference in height after the build.

Opening up the rear façade

In many extensions the client wants to open up most of the existing rear façade, creating one large space between the existing home and the new extension. Structurally this means part of the existing load-bearing structure is removed. The rear façade may carry loads from, for example:

When this façade is removed, those loads must be taken over by a new structure. This is often done with a steel beam or a complete steel portal frame.

Removing a load-bearing wall

Steel portal frame for an extension

When a wide façade opening is made, a single steel beam is not always sufficient. With an almost fully opened rear façade a steel portal frame can be used. This usually consists of:

The structural calculation determines: steel sections, forces, deflection, column loads, connections and the load on the foundation. The forces from a steel portal frame can be considerable. That is why we check not only the steel, but also what is present beneath the columns.

Calculating a steel portal frame

Roof structure of the extension

An extension is often built with a flat roof, but other roof forms are possible. For a flat roof the load-bearing structure may consist, for example, of:

The design assesses, among other things: span, permanent load, snow load, roof covering, insulation, ceiling, services and any green roof or solar panels. When a green roof, roof terrace or heavy installation is desired, this must be included in the structural calculation from the start.

Large sliding doors or glazed rear façade

Many extensions are built with wide sliding doors or a large glazed section on the garden side. This affects the load-bearing structure. Above the sliding doors there must be enough room for:

When almost the entire rear consists of glass, the remaining façade sections are limited. The stability of the extension must therefore also be examined carefully.

Stability of the extension

A structure must not only carry vertical loads. Horizontal loads, for example from wind, must also be transferred safely. In a traditional extension, masonry side walls often provide an important part of the stability. When large glazed sections, narrow piers or open structures are used, additional structural stability may be necessary, for example with:

Connecting new to existing

The transition between the existing home and the new extension deserves extra attention. Several construction functions come together here.

Structural connection

The new structure must connect safely to the existing home without causing unwanted loads.

Thermal connection

Insulation of floor, façade and roof should run through as much as possible to limit thermal bridges.

Waterproofing

The connection between the existing and new roof must be detailed carefully.

Façade connection

Connections must allow for movement and any differential settlement. Good detailing prevents many problems that would otherwise only become visible after completion.

Insulation and energy performance

Thermal requirements also play an important role in a new extension. Depending on the nature of the building project and the applicable Bbl level, requirements may apply to, among other things:

The exact requirements are determined per project. We therefore match the structural build-up to the required insulation thickness and construction detailing.

Ventilation and daylight

When an extension changes the layout or size of habitable rooms, daylight and ventilation may also require attention. A large extension can, for example, affect:

Where relevant, we therefore assess not only the structure but also the constructional consequences of the enlargement.

Drainage and services

An extension is often combined with a new kitchen, toilet or other installation. Timely account must therefore be taken of:

Pipes must not be run through important floor beams, foundation beams or steel structures without a structural assessment. It is therefore wise to establish openings and pipe routes before execution.

Do I need a permit for an extension?

That depends on the situation. Under the Dutch Environment Act a distinction is made between the technical building activity and the zoning-plan activity.

For certain ancillary buildings the technical part can be permit-free. IPLO states conditions for this, including that the structure stands on the ground, is no more than 5 metres high and has no habitable area on a second storey or higher. For the spatial part it must also be checked whether the building plan fits within the local environment plan. Relevant factors may include:

We therefore always advise carrying out the permit check of the Omgevingsloket (Dutch permit portal). Permit-free, moreover, does not mean that no technical requirements apply. The structure must still comply with the applicable rules of the Bbl (Dutch Building Decree).

Structural calculation for the extension

For the structural development we can calculate, among other things:

Where necessary, we also check how the new loads are absorbed by the existing home or foundation.

Structural drawings for contractor and permit

The structural drawings can indicate, among other things:

This gives both the contractor and the reviewing party clear technical information.

Temporary support during the façade opening

When the existing rear façade is load-bearing, it must be temporarily supported during the works. The floor and façade above must not be loaded in an uncontrolled way while the wall is removed. Depending on the situation, the following may be needed:

The final method is matched to the existing structure and the sequence of execution.

Building drawings, structure and permit under one roof

An extension consists of several disciplines that are strongly interrelated. ROMB can combine these parts:

This way the architectural design does not have to be adjusted afterwards because the structure does not fit. The structure becomes part of the design from the start.

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

Our approach

  1. First assessment. You send us the address, available drawings, photos and your wishes for the extension.
  2. Archive and existing situation. We assess the available data on the home and determine which additional information is needed.
  3. Design. The desired extension is worked out architecturally. We immediately consider structural feasibility.
  4. Structural design. We determine the foundation principle, floor system, façade opening, steel structure and roof structure.
  5. Calculations and drawings. The plan is worked out technically for permit and execution.
  6. Permit and additional questions. When a permit is needed, we can guide the application and answer technical questions from the municipality.
  7. Execution. The contractor then has a clear technical set as the basis for the build.

Design first, or find a contractor first?

Both routes occur. For a proper price comparison between contractors, however, it is wise to first have a clear picture of what actually needs to be built. With architectural and structural drawings, contractors can use the same starting points for their quotation. That reduces the risk of one contractor including something different from another. It also means important structural parts are not only discovered during execution.

What do we need from you?

For a first assessment we preferably receive:

Do you not yet have a worked-out design? Then we can also start from your wishes and the existing home.

Frequently asked questions about an extension

How deep may an extension be?

This cannot be determined on the basis of a single national dimension alone. The possibilities relate, among other things, to the environment plan, the location of the plot and the extent of existing buildings.

Is an extension permit-free?

It can be, under conditions. Different rules apply to the technical and spatial building activity. Always check the specific situation via the Omgevingsloket.

Do I need structural calculations if the extension is permit-free?

Permit-free structures must also comply with the technical rules of the Bbl. For load-bearing parts such as foundation, floor, façade opening and roof structure, a structural calculation may therefore be necessary.

Which foundation does my extension need?

That depends on soil conditions, existing foundation, dimensions and loads. Possible solutions include a shallow foundation or a pile foundation.

Can I remove the entire rear façade?

That is often possible, but usually a calculated steel structure is then needed. For large openings a steel portal frame is regularly used.

Can an extension be connected to the existing foundation?

That must be assessed per project. It is unwise to assume without investigation that the existing foundation can take extra load.

Which floor is suitable for an extension?

Common solutions are a beam-and-block combination floor, insulated concrete floor or another self-supporting floor structure. The choice depends on foundation, span and desired floor build-up.

Can I use underfloor heating in the extension?

Yes. It must be considered early in the design because of the total floor build-up and the connection to the existing floor level.

Can the extension be made entirely of glass?

Large glazed sections are possible, but enough must remain for vertical load-bearing capacity and stability. This is assessed structurally.

Can you also handle the permit application?

Yes. In addition to architectural and structural development, we can prepare the required permit documents and guide the application process.

Can you work together with my contractor?

Yes. The drawings and calculations can be coordinated with your contractor, steel fabricator or other parties involved in the execution.

Would you like to have an extension built?

Would you like to know what is structurally possible, which foundation is needed or which drawings you need before requesting quotations from contractors?

Send us the address, a few photos, available drawings and the desired approximate dimensions of the extension. We can then assess which architectural and structural work is needed for your project.

Discuss your extension