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Roof extension · Extra storey

Building a roof extension

Extra living space on top of your existing home

Would you like to create more living space without leaving your home? With a roof extension you can enlarge an existing home with an extra storey or a larger volume on the roof. A roof extension can provide space for, for example: one or more bedrooms, a bathroom, a study, a master bedroom, extra living space, a plant room, or a combination of bedroom and roof terrace.

Structurally, however, a roof extension is far more far-reaching than an ordinary internal renovation. Extra weight is added to an existing home that was not always originally designed for it. That is why not only the new walls and the new roof must be calculated; the existing floor, load-bearing walls, steel structures and foundation may also be part of the structural assessment.

ROMB takes care of the technical preparation of your roof extension: from building drawings and structural calculations to a foundation check and guidance of the permit application.

Discuss your roof extension

What is a roof extension?

A roof extension is an enlargement of the existing building volume on top of or at the level of the roof. This can be, for example:

A roof extension is therefore different from a dormer. A dormer is a relatively limited addition within or on an existing pitched roof surface. With a roof extension the building volume usually becomes considerably larger and even a full new storey may be created.

Can a roof extension be built on any home?

Not automatically. Whether a roof extension is possible depends on two important questions: is it permitted in planning terms? and can the existing home carry it structurally? These two questions must be investigated separately.

In planning terms, factors include, for example:

Structurally, we look, among other things, at:

Only when these aspects are sufficiently clear can a reliable technical solution be worked out.

First examine the existing home

With a roof extension we build on top of an existing structure. A good design therefore does not start with the new storey, but with the home below. Where possible, we investigate:

For older homes it is important to take into account changes that may have been made over the years. The actual situation may differ from the original archive drawings. When important data is missing, additional investigation or an on-site survey may be necessary.

How is the weight of a roof extension transferred?

This is one of the most important structural questions. The loads of the new storey must ultimately be transferred to the foundation:

new roof→ new walls→ existing or new floor→ load-bearing walls / steel→ foundation→ subsoil

Every link in this chain must have sufficient capacity. There is little point, for example, in choosing only lightweight walls when the existing floor or foundation still has insufficient capacity. That is why we view a roof extension as one complete structural system.

Build as lightly as possible

With a roof extension it is usually advantageous to limit the self-weight of the new structure. Roof extensions are therefore often built with relatively lightweight systems, such as:

This can considerably limit the extra load on the existing home and foundation compared with heavy traditional structures. Which construction method is most suitable, however, depends on: spans, floor plan, stability, fire safety, acoustics, insulation, detailing and the existing load-bearing structure.

Checking the existing floor

A new storey is often placed on an existing roof or attic floor. This floor may originally have been designed only as a roof structure, attic floor, storage attic or lightweight usable space. A new living storey can cause different loads. We therefore check, among other things:

If capacity is insufficient, the existing floor can be reinforced or a new load-bearing structure can be designed.

Reinforcing a timber joist floor

In many existing homes the top floor consists of a timber joist floor. When it has insufficient capacity for the new storey, various solutions may be possible, for example:

Which solution is technically and practically best is determined on the basis of the existing situation.

New storey floor

Sometimes it is more practical to build a completely new floor structure for the roof extension. This floor can be built, for example, with timber joists, composite timber beams, steel beams with a timber joist floor, lightweight prefab floors or other lightweight construction systems. Account must be taken of: strength, deflection, vibrations, sound, fire resistance, pipe routing, stairwell opening and floor height.

New walls of the roof extension

In roof extensions the external walls are often built relatively lightly. Timber frame construction is a commonly used solution. Such a wall can consist of:

Structurally the walls must not only carry vertical load, but can also play an important role in the stability of the roof extension.

Roof structure of the new storey

On top of the roof extension comes a completely new roof structure. Depending on the design this can be a flat roof, pitched roof, mono-pitch roof or a compound roof form. Where necessary, we calculate:

We take into account, among other things, self-weight, snow load, wind load, roof covering, insulation, ceiling, solar panels and services. When a green roof or solar panels are desired, for example, it is wise to include this in the structural calculation from the start.

Stability of a roof extension

An extra storey must not only be able to carry vertical load. Wind also causes horizontal forces on façades and roof. These forces must be safely transferred through the new storey and then through the existing building. For this, the following can be used, for example:

Especially with large window openings and open floor plans, stability deserves extra attention.

Large windows and glazed sections

Modern roof extensions often use large windows or glazed sections. This provides a lot of light, but at the same time reduces the available amount of load-bearing wall. The structure around the frames must therefore be carefully designed. Additional timber beams, steel beams, columns or portal frames may be needed. Architecture and structure must therefore be coordinated early on.

Stairs to the new storey

An extra storey usually also means that a new staircase and a new stairwell opening are needed. For the stairwell opening, existing floor joists often have to be interrupted. The load of these joists must then be redirected via a trimmer construction. We assess, among other things:

The position of the stairs therefore directly influences both the architectural and the structural design.

Stairwell structural calculation

Checking the load-bearing walls

The new roof extension should be placed as much as possible above existing load-bearing structures. When new load-bearing walls are not above existing load-bearing walls, additional beams may be necessary to transfer the load. We therefore analyse the structure storey by storey. A new wall on the third floor can, for example, cause load on:

Especially in older homes, this full load transfer is important.

Does the foundation have to be checked?

A roof extension adds extra load to the existing home. It must therefore be assessed whether the foundation also has sufficient capacity. This does not automatically mean that every roof extension needs a new foundation. The assessment depends on:

With a lightweight roof extension the additional load can often be considerably limited.

Roof extension and foundation in Rotterdam

For homes in Rotterdam, foundation investigation is regularly an important part of the feasibility study. Many older homes in the western Netherlands were built on timber or concrete piles. With a roof extension we therefore want to know, where possible:

Available archive drawings and existing foundation investigations can be very valuable here. When insufficient data is available, additional foundation investigation may be needed.

Foundation check for roof extension

What if the existing foundation is insufficient?

When it turns out that the existing structure has insufficient reserve capacity, this does not automatically mean that a roof extension is impossible. Possible solutions may be:

Which measure is suitable is entirely project-specific. That is why it is wise to investigate structural feasibility early in the design process.

Structural calculation for a roof extension

For a roof extension we can, depending on the situation, calculate, among other things:

For renovations of existing structures, the Dutch Building Decree (Bbl) refers to the NEN 8700 series for structural safety. For determining the relevant loads, NEN 8701 is used in conjunction with NEN 8700 and the Eurocodes.

Structural drawings for the roof extension

In addition to calculations, clear structural drawings are necessary to build the design correctly. Depending on the project, we can indicate on them:

Together with the calculations, these drawings form the technical basis for the contractor and the permit process.

Building drawings for a roof extension

For a permit application, structural drawings alone are usually not sufficient. Building drawings are also needed that show the full design. Think of:

ROMB can provide the architectural and structural drawing work within the same project. This means the design and structure can be coordinated directly.

Does a roof extension need a permit?

In permit terms, a roof extension must be assessed differently from an ordinary dormer. The Omgevingsloket explicitly names adding an extra storey as a roof extension and as part of renovating a home. In the permit check, questions include:

For a specific address it must therefore always be checked which permits or other approvals are needed. A roof extension may relate both to the technical building activity and to the rules of the municipal environment plan.

Have the feasibility assessed

Building height and environment plan

Whether an extra storey is permitted in planning terms depends strongly on the local environment plan. This may contain rules about maximum building height, maximum eaves height, number of storeys, roof form, building envelope, position of the façade and use of the building. As a result, a technically feasible roof extension may still not be directly permitted in planning terms. In some cases a deviation procedure or another permit route can be investigated.

Aesthetics and streetscape

With roof extensions, the appearance of the building also plays an important role. The municipality may look, for example, at the proportion of the new storey, the position relative to the existing façade, materials, window layout, roof edge, the relationship with surrounding homes and existing roof extensions in the same street. Existing comparable roof extensions may be relevant as a reference, but do not automatically guarantee that a new plan will be approved.

Roof extension on a corner house

A corner house can offer extra design freedom, but can also be more sensitive to planning assessment. The roof extension is, after all, often visible from several sides. In the design, the front façade, side façade, rear façade, streetscape, termination of the building block and existing architecture then play a greater role. Structurally, a corner house may also have a different stability set-up than a mid-terrace house.

Roof extension on a mid-terrace house

In a mid-terrace house it is often possible to connect to existing load-bearing party walls. That can be structurally advantageous. At the same time, account must be taken of neighbours, party-wall structures, sound, fire safety, watertight connections and execution on the plot boundary. When adjacent homes already have a roof extension, it must also be investigated how these structures were built and whether connecting to them is possible or desirable.

Roof extension and owners' association (VvE)

For an apartment or a divided building, a roof extension cannot be viewed solely from building regulations. The roof and the main load-bearing structure are often communal parts of the building. Therefore, in addition to a building permit, approval from the owners' association (VvE) may also be necessary. Relevant factors may be:

A municipal permit does not automatically mean that private-law approval from the VvE is in place.

Fire safety

An extra storey can also change fire-safety aspects. Depending on the building, attention may be needed for escape routes, stairs, fire resistance of structures, party-wall structures, roof and façade details, smoke detectors and materials. These aspects must be assessed within the overall architectural design.

Insulation and energy efficiency

A new roof extension at the same time offers the opportunity to create a high-quality thermal envelope. Important elements are roof insulation, façade insulation, glazing, window frames, airtightness and thermal bridges. The structural design must therefore leave sufficient room for the required construction build-up. A slender structure can, for example, provide extra internal space, but must still accommodate insulation, services and internal finish.

Ventilation and daylight

New bedrooms and other habitable rooms must have sufficient daylight and ventilation options. The position and dimensions of window frames are therefore not chosen from aesthetics alone. Relevant factors may also be: ventilation capacity, purge ventilation, daylight entry, usable area and the layout of habitable areas. It is therefore wise to include these elements directly in the design.

Watertight connection

One of the sensitive parts of a roof extension is the connection to the existing home. During execution, careful attention must be paid to roof covering, upstands, rainwater drainage, cladding, lead or other water-resistant connections, connections to adjacent roofs and detailing around frames. A structurally correct design must therefore also be feasible in construction terms.

Temporary situation during construction

For a roof extension, part of the existing roof often has to be removed before the new structure is ready. This temporarily creates a different structural situation. Depending on the project, attention may be needed for:

For complex works, an execution or propping plan may be needed.

Structural feasibility first, or design first?

Ideally, both processes partly run in parallel. An architecturally attractive design has little value when it can only be realised with very heavy or costly structural provisions. Conversely, the structure should not unnecessarily limit the design when a better solution is possible. We therefore look at an early stage at the position of load-bearing walls, the stairs, large frames, the steel structure, foundation load and structural heights. This way the design can be optimised before the permit documents are fully worked out.

Our approach

  1. First feasibility assessment. We receive the project address, available drawings, photos and your wishes. We look broadly at the planning and structural points of attention.
  2. Archive research and existing structure. We collect and assess the available data on the existing home, looking for information about floors, load-bearing walls, foundation and previous structural changes.
  3. Roof-extension design. The new storey is worked out architecturally. We immediately consider structural feasibility.
  4. Structural design. We determine the principle for the new floor, walls, steel structures, roof, stability and load transfer to the foundation.
  5. Structural calculations. The relevant existing and new structural components are checked and calculated.
  6. Architectural and structural drawings. The plan is worked out into a coherent technical set.
  7. Permit application. Where agreed, we handle or guide the submission to the municipality.
  8. Answering additional questions. When the municipality or checking structural engineer asks for additional technical information, we can supplement or explain the structural documents.

From permit to execution

A roof extension touches several disciplines. ROMB can coordinate these parts within one project:

This creates one coherent design instead of separate drawings and calculations that only have to be aligned later.

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

Frequently asked questions about a roof extension

Can a roof extension be built on any home?

No. Both the environment plan and the structural possibilities of the existing building must be investigated.

Is a roof extension the same as a dormer?

No. A roof extension is a larger enlargement of the building volume and can, for example, consist of a full extra storey. Different permit rules apply to a roof extension than to a dormer.

Do I need a permit for a roof extension?

For a roof extension, the permit requirement must always be checked for the specific location and design. The Omgevingsloket treats an extra storey as a home renovation and looks, among other things, at enlargement of the building and changes to the load-bearing structure.

Does the foundation always have to be calculated?

With an extra storey the load on the existing building increases. It must therefore be assessed whether a foundation check is needed. How extensive this check must be depends on the existing structure and the available information.

Is a lightweight timber roof extension better?

A lightweight structure can be structurally advantageous because it causes less extra load. Whether timber, steel frame or another system is most suitable depends on the design.

Can the existing roof floor be used as the new storey floor?

Sometimes, but this must be checked structurally. An existing roof or attic floor may originally have been designed for different loads.

Can you check the existing foundation?

Yes. Based on available foundation drawings, archive documents, structural data and, where necessary, additional investigation, we can assess the consequences of the new load.

What happens if the foundation is insufficient?

Then it can be investigated whether the roof extension can be built more lightly, the load can be distributed differently, or reinforcement of the structure and/or foundation is necessary.

Can you also make the building drawings?

Yes. We can provide both the architectural design and permit drawings and the structural calculations and drawings.

Can you apply for the permit?

Yes. We can guide the permit process and prepare the required architectural and technical documents.

Can you work together with my contractor?

Yes. Where necessary, we coordinate structural starting points and execution details with the contractor, steel fabricator or other parties involved.

What do you need for a first assessment?

We preferably receive: project address, photos of the home, existing floor plans, available archive drawings, desired layout, approximate dimensions of the roof extension and available foundation information.

Would you like to have a roof extension built?

Would you like to know whether an extra storey on your home is feasible in structural and planning terms?

Send us the address, available drawings and a short description of your wishes. We can assess which investigations, drawings and structural calculations are needed to develop the plan further.

Have your roof extension assessed