House Extension Planning Permission Ireland: The 45 m² Rule Explained

One of the most searched planning questions in Ireland is also one of the easiest to oversimplify:

How big can I build an extension without planning permission?

The headline number is now 45 square metres.

That number changed on 27 July 2026.

And it is still not the whole rule.

For the wider planning system, start with Planning Permission in Ireland: The Complete Guide.

The 45 m² rule is cumulative

The most common mistake is measuring only the new extension.

Suppose the original house has already been extended by 20 m².

You now want another 25 m².

The new project is below 45 m², but the combined relevant extension area would be 50 m².

That can take the proposed work outside the Class 1 exemption.

The regulations look at the floor area of the proposed extension together with previous extensions constructed after 1 October 1964.

That means the planning history of the house matters.

Rear extensions versus front extensions

The main domestic Class 1 exemption relates to extending a house to the rear, including a conservatory, and to certain conversions of attached garages/stores/sheds at the rear or side.

A front extension is not automatically covered simply because it is small.

If your design changes the front building line or front elevation, check the planning position carefully.

Upper-floor limits

Even where the total extension remains below the 45 m² headline limit, additional limits apply above ground level.

For a terraced or semi-detached house, the floor area of an extension above ground level is limited to 12 m².

For a detached house, the upper-floor limit is 20 m².

These sit within the overall Class 1 conditions.

So a 35 m² two-storey extension is not assessed only by asking whether 35 is below 40.

You have to examine how much of the extension is above ground level and the type of house.

You must leave enough private open space

The extension must not reduce the relevant private open space within the curtilage below 25 m².

This matters particularly on:

• small urban plots;
• terraced houses;
• homes that already have an extension;
• and sites where sheds, garages or other structures occupy much of the rear.

Measure the actual remaining private space rather than assuming the garden “looks big enough”.

Height matters

Class 1 also contains height restrictions.

The permitted height depends on the form of the extension and relationship to the existing house.

This is one reason a very tall single-storey extension, raised roof or unusual parapet can fall outside the exemption even when the floor area looks compliant.

If the design is near a height limit, use the actual regulatory wording rather than a builder’s rule of thumb.

Windows can affect the exemption

Window placement is another condition.

Upper-floor windows close to boundaries can raise overlooking and planning issues.

The regulations contain specific conditions around certain windows at upper-floor level.

Do not assume a design is exempt solely because the walls and floor area fit.

Changes to glazing can matter.

Can I build right up to the boundary?

Boundary relationships matter differently depending on the form of the extension.

For upper-floor extensions, separation from boundaries is specifically relevant.

Even where planning exemption is available, separate private-law issues can arise around:

• party walls;
• ownership;
• access;
• overhanging gutters;
• foundations;
• easements;
• and rights of way.

Planning exemption does not give you a right to build on land you do not own.

Does an old extension count?

Extensions built after 1 October 1964 can be relevant when calculating the Class 1 total.

That includes extensions that may themselves have received planning permission.

This surprises people.

A previously permitted extension can still affect how much additional exempt floor area remains.

Before designing a new extension, establish:

• the original footprint;
• later extensions;
• dates if available;
• planning reference numbers;
• and the existing measured floor areas.

Does a conservatory count?

Yes.

The Class 1 description specifically includes a conservatory.

Calling a glazed room a conservatory does not create a separate exemption allowance.

It is part of the extension calculation.

What about converting an attached garage?

The Class 1 exemption can include conversion for use as part of the house of certain attached garages, stores, sheds or similar structures at the rear or side.

The key phrase is use as part of the house.

A conversion into a separate dwelling, commercial premises or independent letting unit can raise a different planning question.

Use, not just construction, matters.

What changed on 27 July 2026

The 2026 residential exempted-development regulations came into force on 27 July 2026.

The Class 1 rear-extension limit increased from 40 m² to 45 m².

The change does not remove the other conditions. Previous extensions still count, upper-floor limits still matter, and the design still has to satisfy the rest of the exemption.

What if my extension is 41 m²?

If the current exemption limit is 45 m² and your proposal falls outside it, the ordinary approach is to seek planning permission rather than assuming the excess is too small to matter.

Planning thresholds are not approximate.

A design that is one square metre over a legal exemption threshold does not become exempt because the difference feels insignificant.

A professional may be able to redesign the project, but do not build first and hope the difference is ignored later.

Can I split an extension into phases?

Do not assume that building 25 m² now and another 25 m² later creates two separate 45 m² allowances.

The rules are cumulative.

Previous extensions are specifically part of the calculation.

What if the house was already extended before I bought it?

This is extremely common.

You may have no idea what work previous owners carried out.

Before relying on exempted development:

1. search the planning register;
2. obtain old drawings where available;
3. have the existing building measured;
4. compare the present footprint with historic records;
5. ask your architect/engineer/planner to assess the position.

If uncertainty remains, consider a Section 5 declaration.

Do I need planning permission for an attic conversion?

An attic conversion is not automatically the same thing as a rear extension.

Internal works may be exempt in planning terms, while dormer windows, roof alterations, front-facing changes or other external works can create planning issues.

Attic and roof works have their own planning issues. See Attic Conversion & Dormer Planning Permission Ireland.

What about a protected structure?

Do not apply the ordinary 45 m² checklist blindly to a protected structure.

Works that materially affect the character of a protected structure can require planning permission.

A Section 57 declaration may be relevant for determining which works would or would not materially affect the character of the structure.

Likewise, external works in an Architectural Conservation Area can have additional constraints.

Planning exemption does not mean building-regulation exemption

Even if the extension is exempt from planning permission, the work must still comply with applicable building regulations.

Building-control requirements can also apply.

For extensions over 40 m², commencement-notice documentation is particularly important.

The planning question and the building-control question are separate.

When should I ask for a Section 5 declaration?

A Section 5 declaration is particularly useful where:

• the previous extension history is unclear;
• the proposed design sits close to a threshold;
• there is disagreement between professionals;
• you are buying or selling a property with an undocumented extension;
• a lender or solicitor needs stronger evidence;
• or the proposed use is ambiguous.

The local authority can formally determine whether the development is or is not exempted development.

Read Section 5 Declaration Ireland.

If permission is required

The normal statutory application fee for a domestic extension is currently €34.

The wider cost will usually include professional drawings and possibly specialist reports.

A straightforward application generally targets an eight-week local-authority decision period where no further information is required, followed by a four-week appeal window.

Read Planning Application Process Ireland.

Buying a house with an extension

If you are buying a house with an extension and cannot find planning permission, do not jump directly to the conclusion that it is unauthorised.

It may have been exempt.

But somebody needs to establish why.

Your solicitor, surveyor, architect or engineer may look for evidence such as:

• original house layout;
• measured extension area;
• construction date;
• planning history;
• compliance with the relevant exemption rules;
• and certificates/opinions on compliance.

A Section 5 declaration can be useful where the position is not clear.

Common mistakes

“It is only 30 m²”

That ignores previous extensions.

“The neighbour built the same thing”

Their planning history and site can be different.

“It is a conservatory, so it does not count”

Conservatories are included in the Class 1 extension rules.

“The limit is still 40 m²”

That was the old rule. The headline limit increased to 45 m² on 27 July 2026.

“If it is exempt from planning, there are no regulations”

Building regulations and building control are separate.

“I can fix it with retention later”

Retention is not guaranteed.

Frequently asked questions

What is the current extension limit without planning permission?

The headline Class 1 limit is currently 45 m², subject to all other conditions.

Do old extensions count?

Relevant extensions constructed after 1 October 1964 can count toward the total.

Is the limit 45 m² now?

Yes. The headline Class 1 rear-extension limit increased from 40 m² to 45 m² on 27 July 2026, subject to the full set of exemption conditions.

How much garden must be left?

At least 25 m² of private open space must remain to the rear under the Class 1 conditions.

Can a two-storey extension be exempt?

Potentially, but smaller upper-floor limits apply: 12 m² above ground level for terraced/semi-detached houses and 20 m² for detached houses, within the overall rules.

What if I am unsure?

Consider professional advice and a formal Section 5 declaration before building.

The bottom line

The right way to test a domestic extension is not:

“Is it below 45 m²?”

It is:

“Does the entire proposal, including the history of the house, satisfy every condition of the current exemption?”

If the answer is uncertain, resolve it before construction.

That is much cheaper than trying to prove compliance after the work is finished.