Modular homes: planning, building rules and your budget

Check planning, building compliance and the full project price before ordering a modular home. Factory manufacture does not create a planning exemption or remove building regulations.

A modular home is built in sections away from the site, then delivered and finished where it will stand. A house on its own plot normally needs planning permission. A qualifying rear-garden house may use the Class 3A exemption introduced on 27 July 2026. That exemption applies to detached houses meeting its conditions, however they are built. Have your adviser check the regulations and your council’s guidance against the proposed design before you rely on it.

Check permission before ordering

Factory manufacture is not itself a planning exemption. For a new dwelling on its own plot, work on the basis that planning permission is needed unless your planning adviser identifies an applicable exemption. The rear-garden Class 3A exemption has a specific location and conditions; it does not extend to any small home placed on any site. Follow the usual route in planning permission for a one-off house, including site suitability and a wastewater system if you are not on a mains sewer.

The Class 3A exemption: a detached home behind your own house

Statutory Instrument (S.I.) No. 340/2026 came into operation on 27 July 2026. It adds Class 3A to Part 1 of Schedule 2 of the Planning and Development Regulations 2001: “the construction, erection or placing of a detached house in the rear garden of a principal house”. The exemption applies only to a house started and finished between 16 July 2026 and 31 December 2030. Check all 20 conditions with your adviser. The main checks are grouped below.

Check how the home will be used

• The home may only be occupied in conjunction with the main house and cannot be sold or subdivided separately.
• The principal house must be your sole or main residence when work starts.
• It cannot be used for short-term letting.
• It cannot be used where the principal house has been subdivided.
• It cannot be temporary, such as a caravan or mobile home.

Measure the house and garden

• The floor area must be at least 32 m². Together with any other exempt garden buildings (Class 3 structures, such as sheds) already in the garden it cannot exceed 45 m².
• Height is capped at 4 m with a tiled or slated pitched roof, or 3 m otherwise.
• The main house must keep at least 25 m² of private open space reserved for its occupants. Have your adviser show the qualifying private open space on a measured plan; do not assume shared access or parking counts.
• The home must be at least 0.6 m from any wall or party boundary, and any window at least 0.6 m from the boundary it faces.

Check access and service capacity

• No new vehicular or pedestrian access onto a road may be built under this class.
• Independent pedestrian and/or wheelchair access to the new house must be provided inside the curtilage of the main house.
• There can be no separate utility connection, including water or wastewater.
• If you are on a septic tank or other non-piped system, the new home must not encroach on an approved percolation area. The system must have enough capacity for the extra load and comply with the Environmental Protection Agency (EPA) Code of Practice. No additional wastewater treatment units may be installed under this class.

Notify the council before starting

• You must notify the planning authority at least 14 days before starting, say you are using the exemption, confirm the site meets the requirements, and state the reason. The notice must include the location and Eircode.
• Planning authorities keep a record of notifications and report the numbers to the Minister each year.

If you cannot meet the conditions, do not rely on this exemption. Follow the planning application process for work needing permission, and read about retention permission if something was built without it.

Class 3A is a planning exemption only. You still need a site that can take the extra load, a design that meets building regulations, and insurance that covers a second dwelling. If a council later decides a condition was not met, the home is unauthorised development, and you may need to seek retention permission, which is not guaranteed.

Building regulations still apply

Class 3A expressly preserves building regulations, including fire safety, and building control legislation. The 0.6 m planning boundary condition is not proof that a particular wall, window or roof design complies with fire requirements. Have a competent designer check the proposed layout and construction before ordering a unit.

Agree the building control route before work starts. A new single dwelling on its own site can opt out of the usual route, where an architect, surveyor or engineer inspects the build and signs it off, by lodging a declaration with the commencement notice. Confirm eligibility for your project rather than assuming a second dwelling in a garden qualifies. Opt-out does not remove the owner’s duty to ensure compliance, the commencement notice or the required design information and builder assignment.

Where statutory certification applies, arrange an assigned certifier, inspections and completion-certificate registration before occupation. If opting out, agree the professional inspections and evidence you still need, including anything required by a lender or insurer. A supplier’s generic certificate or product brochure is not a substitute for evidence that your completed home complies. Read fire safety and disability access before choosing a design.

Get the supplier’s responsibilities in writing

Define the package in writing. A quotation for a shell and a quotation for a completed home are different products. Ask who takes responsibility for the interfaces between the factory unit and the foundations, drainage, electrical installation and weatherproofing. Confirm who repairs damage during transport or lifting, and who inspects concealed work before it leaves the factory. Before you sign, ask:

1. What is included: shell only, or fit-out, delivery and crane?
2. Who designs it and who signs the design for compliance?
3. Who builds the base, and who connects services?
4. Is the home designed to current nearly zero energy building (nZEB) and fire requirements?
5. What warranty and structural defects cover will a lender and insurer accept?

Compare the whole project cost

Get a costed design and quotations covering the same work before comparing modular and traditional construction. Include ground investigation, foundations, access for delivery and lifting, service connections, wastewater capacity, professional work, fit-out, taxes and contingency. Ask which sums are fixed, provisional or excluded, and when payments become due relative to manufacture, delivery and inspection.

Check finance before paying a deposit. Tell the lender whether this is a standalone house or an exempt rear-garden dwelling, what security is available and when money will be needed at the factory. Do not assume a standard self-build drawdown arrangement will fund off-site manufacture or that a planning exemption guarantees mortgage eligibility. See self-build mortgages, Help to Buy and insurance lender requirements, and plan the budget and contingency.

Common mistakes

• Treating “no planning” as “no rules”. Building control, fire safety and certification still apply.
• Assuming a supplier’s price covers the base, services, wastewater and fit-out.
• Missing the 14-day notification, or starting before the main house is your main residence.
• Planning a separate water or sewer connection, which the exemption does not allow.
• Counting only the new home’s size and forgetting existing Class 3 sheds, which count toward the 45 m².

For related methods, see timber frame, block and insulated concrete formwork (ICF), since many modular systems are timber or steel frame.