Renovation work in progress of period home

How to control the cost of a house extension and refurbishment.

The cost of a house extension and refurbishment is usually the largest investment an owner makes in their lifetime after the house purchase. But there are long-term benefits including reduced energy costs and the comfort of your home for many years. The building costs depend on several factors including house size, construction methods, materials, finishes and site context. To control the costs of a house extension, it is important to prioritise your aspirations and compromise on less important wishes. But first let’s take a look in detail at what costs are involved and how to reduce the cost of your house extension.

What are the costs of extending and refurbishing your home?

Building costs:

The size and scale of the project is the most obvious primary cost. The RIAI Cost Guidelines for Domestic Works 2019 give costs ranging from €1,900/m2 for a basic single of 2 storey extension. The figure rises to €3400/m2 for a single storey or 2 storey extension with renovations to existing house. This is where the new work is to a specific design and/or with high quality finishes. Domestic work costs have increased since 2019 due to supply chain difficulties as well as fuel and energy costs.

The site context is another item which can affect cost as land may need to be flattened or excavated to prepare the site. Trees, drains and buried services can present costly obstacles.

Extension to cottage in Cork requires levelling of land
Extension to cottage in Cork

If you are adding an extension you also need to consider replacing the boiler to heat the additional space.   The Nearly Zero Energy Building (NZEB) building standards requires that renewable sources must provide 20% of the primary energy use. Heat pumps can be installed in almost any home; however, they work best in homes which are properly insulated. In older houses this may not be possible due to poor insulation. A Protected Structure may require a larger heat pump to make up for the heat loss. This could potentially cost more money to install and to run.

Consultants’ fees

The Architects fees which are agreed with the client in writing will vary depending on the requirements of each project. This may be on the basis of a percentage of the total project cost (excluding VAT on construction costs) or a fixed price lump sum or a time charge basis. There is an additional cost also for an Assigned Certifier (usually the architect). The assigned certifier inspects the works to ensure compliance with the requirements of the Building Regulations and certifies the works. Other consultants may be required such as an engineer to advise on projects with structural or site related complexity. A quantity surveyor may be useful on very large-scale jobs. Other fees could include legal fees for party wall agreements or neighbours’ disputes.

Planning Costs

The costs for planning permission in Dublin vary according to the type of work being undertaken. At the time of writing, the fee for a planning application for maintenance work costs €34 euro and €65 for a residential planning application. Planning permission is only required for an extension which is more than 40 square metres (this includes new and any existing extensions) or there is a change how the land is used, like converting a garage to a place of business. This is known as a “material change of use”. There are various other conditions for extension under 40 square metres which we will advise on.

How do you minimise the cost when extending and refurbishing your home?

The brief and the budget:

Given that the area of the extension to your home has a major impact on the budget, it is important to maximise the existing space. The extension may make other areas of your home redundant. So, you may want to think about how you can use that area efficiently and incorporate it into your extension. You also want to ensure the costs of the extension does not outweigh the profit gained when selling.

Construction type:

The most expensive elements in the build are usually the structural steel elements. Removing structural walls and replacing them with long spanning beams is costly. But sometimes the long-term space benefits outweigh the initial cost.

Windows are another expensive element and are more expensive than traditional materials.  Standard size openings and skylights are less expensive than bespoke windows. Windows are priced in advance of the tender and are included in the tender documentation as a fixed price cost.

Windows to extension to house in Ranelagh
Windows to extension to house in Ranelagh - butterfly roof catches the light.

Timber frame construction is faster and has significantly lower labour costs compared to traditional brick-built extensions.  This form of construction still offers a high quality, energy-efficient build.  However experienced builders are required to ensure a good finish and proper fire resistance. Care must be taken to avoid exposure to moisture and timber is more susceptible to decay. Timber provides a poor level of sound insulation as compared with concrete or masonry.


We provide a detailed drawings which allow you to obtain prices for finishes such as wooden flooring, bathroom tiles or carpets.  Alternatively, we can also provide you with budget estimates of a spend on the various finishes. These are included as Prime Cost (PC) sums in the tender documentation. This allows you to adjust your budget if you save on one item and spend on another. You may like to select your preferred kitchen brand in which case the kitchen company designs and costs the kitchen. A less expensive option may be for us to prepare the drawings which you may tender to local kitchen joiners. Just remember that if you purchase any items yourselves you are subject to 23% VAT whereas the builder is only subject to 13.5%. You may also be responsible for delays due to the late arrival of materials.

As architects we can advise you on what finishes will provide maximise the effect and yet minimise the cost. But you also need to consider the quality of the finishes in terms of durability. Lifestyles can also impact on finishes such as whether you have a pet or a young family.

Bespoke Kitchen in house in Clonskeagh
Bespoke Kitchen in house in Clonskeagh

What should I allow for a contingency fund?

It is important to decide and choose as much as possible before starting on site to avoid additional costs. So, it is important that all parties involved in the decision-making process attend the key meetings. The contract sum is agreed, and a contract signed with the builder prior to commencement of the works. We recommend that you allow for a contingency fund of at least 10-15 % as it is inevitable that some changes will be necessary for alterations and unforeseen items.  Variations to the contract must be priced and agreed in writing with the client.

When do I pay the builder for the work?

In order to avoid delays, it is necessary that the mortgage has been drawn down to ensure that you have sufficient funds in your account to meet the payment deadlines. The architect periodically certifies the works and payments must be made (minus 5 or 10% retention) within 7 working days. Projects can get suspended while a client has to wait for the mortgage top-up.

When is the best time of year to extend and refurbish my home?

Aiming for a spring/early summer build might save you money. You’ve a better chance of avoiding delays due to bad weather (though not always in Ireland!), which would push up labour costs as well as living costs if you’re renting elsewhere during construction. Try to move out of your home for the critical phase as least. If the builder has to work around you and keep the site safe and clean every evening, you will pay for it.

Should I discuss the house extension with my neighbours?

Hopefully you will have a good relationship with your neighbours but whether or not is important to consult them regarding our plans before you start work. Issues with neighbours at a late stage can cause delays to the project and increase cost. With a party wall a photographic condition survey will need to be carried out and details of the work to any party walls agreed with your neighbour

Extending your home is a major undertaking and considerable investment. A good Architect can save you money on a project, as they will research the possible alternatives and solutions to suit your budget. They will help you set a viable brief and budget and monitor it during the course of the contract.  When you consider the endless years of comfort and enjoyment you will have in your home, it is a worthwhile venture.