How do you design an expandable modular home?

In shortAn expandable modular home is prepared before purchase: a simple plan, services left ready, a checked structure and early planning checks. The AIIA website does not describe extensions by adding modules on its 15 models, so ask for it in writing in the quotation. In an urban zone, an extension of up to 40 m² generally needs a prior declaration, unless the total area exceeds 150 m².

Key figures

40 m²: extension size above which a building permit is required in an urban zone of a local plan; a permit may apply from 20 m² if the total area exceeds 150 m² (Service-Public, sheet F36777)
20 m²: equivalent threshold outside urban zones (Service-Public, sheet F36777)
150 m²: floor area after works above which an architect becomes compulsory for a permit (Service-Public, sheet F20568)
1 month: review time for a complete prior declaration (Service-Public, sheet F17578)
6 to 8 weeks: workshop manufacturing of an AIIA model (AIIA)

*By Yannick Akoka, co-founder of AIIA · published 7 November 2026*

You are torn between a home that suits today and one that will follow your family, your home working or your rental plan. An expandable modular home cannot be improvised after delivery: services, structure, plan and planning permission are decided at the start. This article gives the method and the official extension thresholds. It also says what the AIIA website lets us state, or not, about extending its models. It helps you decide whether to buy bigger now or to grow in stages.

What is an expandable modular home?

An expandable modular home is a home whose plan, services and structure were designed to take an extension, an extra storey or an outbuilding later. The word "modular" only describes workshop manufacturing in modules.

A modular home is therefore not expandable by nature. It is so only if the extension was anticipated and confirmed in writing. For the whole buying journey, see the complete guide.

Professionals distinguish three forms of growth:

horizontal extension, on a reserved part of the plot;
adding a storey, which is possible only if the structure and foundations allow it;
an outbuilding, a small building attached or separate, for example an office or a home for a relative.

Each form has its own structural, services and permit constraints. The next sections detail them.

What does the AIIA website say about extending its models?

The AIIA website does not describe extensions by adding modules on its 15 models. It offers personalisation through options on a "fixed base": the structure, insulation and joinery stay fixed (AIIA FAQ, read on 7 November 2026).

The Our approach page mentions "evolving homes", "transformable, and even movable", without saying what can be changed. The plans on the model pages are presented as indicative and not meant for execution.

For you, this means one thing: do not buy a promise of expansion that the quotation does not write down. If extending matters to your project, ask AIIA through the Contact page what is possible on your chosen model, on what conditions and with what guarantees.

A compact model can already support a staged approach. According to the page for Line (40 m², from €52,000*), it suits "a first home or an outbuilding at the bottom of the garden". That is a use, not an extension. Housing a relative is covered in Housing a relative at the bottom of the garden: what rules?.

AIIA Line, 40 m² modular home: exterior view of a possible outbuilding at the bottom of the garden, 3D render — 3D render of Line (40 m², from €52,000*), presented on the website as a first home or an outbuilding at the bottom of the garden.

Which plan choices make a home expandable?

An expandable plan stays simple, with few interior load-bearing walls and rooms that can change use. The more the load-bearing structure is limited to the perimeter, the easier it is to rearrange the inside or to connect an extension.

Professional advice converges on a few principles (Maison Plans, February 2026; BÂTI85, August 2025; UDAP 05 / CAUE 05 advice sheet, 2024 edition):

Plan choice	What it allows	What you check
Load-bearing walls on the perimeter, light partitions	Joining or separating rooms	Which partitions are not load-bearing
One or two neutral rooms	Switching from office to bedroom	Floor area, daylight and access
Through living room	Adding an extension on one side	The reserved façade and the orientation
Staircase position planned	Adding a storey without upsetting the ground floor	Ceiling height and possible stairwell
Future openings identified in the façade	Connecting the extension without breaking the volume	Who makes the junction, and at what cost

The UDAP 05 / CAUE 05 sheet also reminds us that an extension must respect the original volume: the initial building stays dominant, without fragmenting volumes. An over-partitioned plan makes that junction harder. An architect can support this work: read Architect: how to bring a modular home into a project?.

Which services and structure should you plan from the start?

You should plan service connections and check the structure's capacity before buying. Adding a connection or strengthening a structure afterwards can cost more and means reopening finished work.

On services, Maison Plans recommends accessible provisions:

spare modules and capacity in the electrical panel;
accessible ducts for future links;
grouped drain stacks, so that future wet rooms stay close together.

On structure, adding a storey requires knowing the capacity of the foundations and floors. The UDAP 05 / CAUE 05 sheet asks for the existing structure to be assessed before any storey is added. A builder quoted by Construire sa Maison claims that oversizing concrete pads costs less than adding foundations later: this is a company claim, to be checked by a structural engineer.

Connections stay outside the AIIA offer: they are done with a local professional. Foundations and their sizing are covered in Does a modular home need foundations?.

AIIA, the five steps to prepare an expandable modular home, from future need to siting, infographic — Five steps to prepare an extension before buying, from defining the need to siting.

What permit is needed to extend later?

For an extension, the formality depends on the area created and on the local plan zone. According to Service-Public (sheet F36777), a prior declaration is enough up to 40 m² in an urban zone of a local plan, and up to 20 m² outside it or in a protected site; above that, a building permit is required. The same sheet notes that a permit may apply from 20 m² in an urban zone if the total area exceeds 150 m².

The calculation uses the footprint or the floor area. According to the glossary of sheet F36777, the footprint is the vertical projection of the volume, overhangs included. The floor area adds up closed and covered levels with more than 1.80 m of ceiling height.

Situation	Formality	Legal review time for a complete file
Extension up to 40 m², urban zone	Prior declaration	1 month, 2 in a protected area (F17578)
Extension above 40 m², urban zone	Building permit	2 months for a single-family house, 3 in a protected area (F1986)
Extension up to 20 m², outside urban zone or protected site	Prior declaration	1 month, 2 in a protected area (F17578)
Extension above 20 m², outside urban zone or protected site	Building permit	2 months for a single-family house, 3 in a protected area (F1986)

An architect is never compulsory for a project under prior declaration. For a permit, one becomes compulsory if the floor area exceeds 150 m² after works (Service-Public, sheet F20568). These rules were read on 7 November 2026.

These thresholds apply to an extension.

A separate outbuilding is a new building: according to sheet F1986, a permit is required for a building of more than 20 m², so in principle for a 40 m² Line, to be confirmed at the town hall.

The local plan, a protected site or the area already built can also change the answer. The times are legal review times, not a promise. In a protected area they are longer: 2 months for a prior declaration, 3 months for a single-family house permit (F17578, F1986). The town hall confirms the applicable regime: ask it before fixing the plan. Siting details are in Modular home: is a building permit needed?.

Is it better to buy bigger now or to grow in stages?

Growing in stages limits the initial outlay, but buying bigger now avoids a second building site. The right choice depends on how certain your future need is, on your budget and on what the quotation allows.

The table divides the 2026 catalogue starting price by floor area. This calculation is a comparison hypothesis, excluding land, connections and options. It says nothing about the real cost of an extension, which AIIA does not publish.

AIIA model	Area	Starting price	Price per m² (calculated)
Axis	65 m²	€57,000*	about €877*/m²
Block	73 m²	€94,000*	about €1,288*/m²
Line	40 m²	€52,000*	€1,300*/m²
Level	48 m²	€79,000*	about €1,646*/m²

\* Prices exclude tax, AIIA 2026 catalogue, standard finish.

Two readings emerge. If the future area is certain, compare the larger model with the initial model plus an extension quoted separately. If it is uncertain, a separate outbuilding, such as a Line at the bottom of the garden, is another route: it leaves the house untouched, but in principle needs its own permit.

Level (48 m²) is presented on its page as two storeys on a small footprint. The subject is covered in Two-storey modular home: what real gain?.

AIIA, starting price per m² of four models: Axis 877, Block 1,288, Line 1,300, Level 1,646 €*, infographic — 2026 catalogue starting price divided by floor area: a comparison calculation, excluding land, connections and options. * Prices exclude tax.

Which pitfalls should you avoid when planning an expandable home?

The most common pitfall is planning only the interior, without the structure or services. The other mistakes noted by professionals are largely prevented by a written specification.

Not checking the local plan: an extension planned on land that cannot be built on, or too close to a boundary, may be refused.
Multiplying interior load-bearing walls: the plan stiffens and the extension becomes costly.
Planning a wet room upstairs without drainage below (Construire sa Maison).
Forgetting daylight: a room fitted out later without enough windows remains unusable.
Fragmenting the volume: an extension that distorts the house raises an integration problem (UDAP 05 / CAUE 05 sheet).
Relying on a verbal promise: only a written mention in the quotation lets you prove it.

Daylight matters as much as area. The article Does natural light make a small space feel bigger? shows how to plan it from the start.

How do you prepare a quotation request for an expandable home?

Prepare five written questions, then send them with your project. AIIA answers within two working days.

Which extension is possible on this model: horizontal, extra storey or outbuilding, and in what form?
Which elements stay fixed: structure, insulation, joinery (the website says they do)?
Which service provisions can be planned from delivery?
Which guarantees remain after an extension: 10 years on the structure, 5 years on waterproofing, 3 years on installations?
Which siting plan leaves room for the extension, with the access needed?

AIIA is neither a builder nor an installer: any extension needs an installer of your choice, whom the team can help you find. A site study is available on request. The guarantees are detailed in What guarantees come with a modular home?.

Where should you start with an expandable modular home?

Start by deciding what you want to be able to add, then have feasibility checked before choosing the model. This order avoids buying a house that cannot be extended.

The method has five steps: define the future need, check the local plan with the town hall, get in writing what AIIA provides for the chosen model, plan services and structure, then fix the plan and siting. For a general framework, read Designing a modular home: which method?.

Ready to move on? Browse the models in the collection or start the configurator, then request your quotation with your five questions.

The AIIA view

At AIIA, we prefer a written answer to a promise. The website presents a fixed base, options chosen in the configurator and indicative plans: a model's expandability must therefore be confirmed in the quotation, not assumed. AIIA is neither a builder nor an installer. An extension needs a local professional, planning permission and the town hall's confirmation. Our advice: if enlarging matters to you, ask before choosing the model, and keep the answer in writing.

AuthorYannick AkokaYannick Akoka co-founded AIIA with a simple conviction: remarkable architecture should never be a privilege. He oversees the design of every collection.About

Sources

Frequently asked questions

Can an AIIA modular home be extended after delivery?

The AIIA website does not describe extensions by adding modules on its models. Whether it is possible depends on the model and is confirmed in writing in the quotation, with the AIIA team and an installer.

Does a 30 m² extension need a permit?

In an urban zone of a local plan, a 30 m² extension generally needs a prior declaration, not a permit; outside urban zones or in a protected site the threshold is 20 m², so a permit (Service-Public, sheet F36777). If the total area exceeds 150 m², a permit may apply: the town hall confirms.

Do you need an architect for an extension?

Not for a project under prior declaration. For a permit, an architect becomes compulsory if the floor area exceeds 150 m² after works (Service-Public, sheet F20568).

Is a two-storey home more expandable?

Not by nature. It takes a small footprint, but adding a storey requires checking the structure and foundations. A storey already shown on the plan is easier to validate.

Is a separate outbuilding better than an attached extension?

A separate outbuilding avoids touching the existing structure, but it is a new building that needs its own planning file. An attached extension keeps the house unified but needs a careful junction and extended services.