Building Advice: What is the extra cost of adding an open-plan mezzanine

The pros and cons of an open-plan living space with double height

Hi Kieran,

We are considering building our own home and I want an open-plan design. I’d love to have a mezzanine overlooking the living space, but my wife thinks it will add significantly to the construction cost. Is this true? 

Thanks Martin

Hi Martin, 

Thank you for your question. One of the greatest advances in house design in the last 20 years is the open-plan living space. This came about as more architects became involved in the house-building sector.

Architects concentrate much of their efforts on introducing light, flow, and good-quality space into our homes. The most important space is the day living space: The kitchen, dining, family living area.

If you have enough room and either access to wonderful light or perhaps a rural or coastal view, you might rightly wonder what extra advantage you could gain from these natural features. One such opportunity is, perhaps, a double-height space or mezzanine.

But what does this entail, from a construction point of view, and does it cost more?

The first thing you will need to consider, outside of a raft of building regulations, is how this space will actually be built.

You would be forgiven for thinking that a double-height space saves on first-floor construction costs, which it does, but what you save here you quickly lose on introducing the cost of internal scaffolding: If you have a first floor, you won’t need the scaffolding, as the first floor provides the majority of the access.

Next, you must consider how to frame this double-height and mezzanine space. You will, undoubtedly, need structural steel to strengthen the edges and provide general support to the open space, as this cannot now be braced together with a first-floor structure.

Then, you need to consider how you insulate and roof this space.

Kieran McCarthy: 'You would be forgiven for thinking that a double-height space saves on first-floor construction costs.'

If you are considering a space such as this, there will be a high level of architecture, with an interesting vaulted or mono-pitch/flat roof above.

In both cases, you will likely need a bespoke insulation design to suit.

As a rule, the most cost-effective roof is pitched, with a flat ceiling below, where you can apply a layer of quilt insulation, but when you deviate from this (as you appear to be proposing), you will likely need a rigid insulation and a more bespoke roofing design.

Finally, and not inconsequential to your budget, is the glazing.

Again, I will have to assume that if you are designing a double-height mezzanine floor, you will be seeking double-height glazing, basked in natural light and pouring over your garden views. This double-height glass will need further structural steel to support the building adjacent and the window frames, and, indeed, the glass itself will likely need to be commercial-grade, to work in this setting.

As we wrap us this discussion, you pass neatly into what bank managers call the ‘Ah, Sure’ list. This is: “Now that we have gone this far, ‘ah, sure,’ we might as well put a nice, glass balustrade into the mezzanine section; ‘ah sure’, we should really buy a nice kitchen to set it off; ’ah sure’, we need a crystal chandelier to accentuate the double-height space”... you get the picture.

So, in truth, once you deviate from the norm, you end up turning up the volume on a variety of different costs.

But, when you wake up early on a Saturday morning, when everyone else is still asleep, and you sip an Americano, while basking in early morning light and drinking in the view, will you really mind? I don’t think so.

• Civil engineer Kieran McCarthy is founder of, and design and build director with, KMC Homes. He is a co-presenter of the RTÉ show, ‘Cheap Irish Houses’.