Most people come to us with a rough idea of the space they want and no strong view on whether it should be one storey or two. It is a bigger decision than it looks, because the two options differ in cost per square metre, planning treatment, how long you live on a building site, and what you get back when you sell. Here is how the choice actually plays out on Ayrshire houses.
A single storey rear extension carries the full cost of foundations and a new roof for one floor of usable space. Add a second storey and you spread those same foundations and that same roof over twice the floor area, so the upper floor typically costs noticeably less per square metre than the ground floor beneath it.
As a rough guide for this part of North Ayrshire, a straightforward single storey rear extension tends to land somewhere around £1,800 to £2,600 per square metre for a decent standard finish, with kitchens, bifolds and structural steel pushing it higher. A double storey is often in a similar bracket overall but gives you roughly double the space for well short of double the money. Every job is different, though, and the ground you are building on matters more than any average figure.
In Scotland, many single storey rear extensions on a detached or semi detached house can go ahead under permitted development, subject to limits on depth, height, how much of the garden you cover and how close you build to a boundary. That saves time and a planning fee, though you still need a building warrant from North Ayrshire Council and the warrant is the part that takes real effort.
Two storey work is far more likely to need full planning permission. Upper floor windows raise overlooking questions with neighbours, and there are rules on how close a two storey element can sit to a boundary. If you are in a conservation area, in a listed building, or your permitted development rights were removed when the estate was built, assume you need consent either way. Budget eight to twelve weeks for a planning decision and allow time on top for the warrant.
A single storey build can often be kept largely outside the existing walls until near the end, so you stay in the house with a reasonable life. A double storey is a different experience. The roof gets opened up, a bedroom wall usually comes out for the new opening, and you will be living around scaffolding and dust for longer. Many families move out for a few weeks of it.
Going up also leans harder on what is already there. The existing foundations and walls may need underpinning or strengthening to carry the extra load, and that is discovered once trial holes are dug, not before. On a lot of the post war housing around Kilwinning and Irvine the foundations are shallower than people expect. It is worth paying for trial holes early rather than finding out halfway through.
If what you want is a bigger kitchen and dining space that opens onto the garden, and your bedroom count already works, a single storey is usually the better answer. It is quicker, easier to get consent for, and the open plan ground floor is what buyers in this area respond to.
If you need a third or fourth bedroom, or a bedroom with an ensuite, and you have the garden depth and the boundary clearance, a double storey is the more efficient use of money. Be realistic about ceilings on value, though. There is a price point beyond which houses in a given street do not sell, and spending £90,000 on a four bedroom conversion in a street of three bedroom semis may not come back to you. A local estate agent will tell you that ceiling for free.
No. A single storey rear extension of a typical size usually runs around twelve to sixteen weeks on site, and a comparable double storey tends to be in the region of eighteen to twenty six weeks. The shared groundworks and roof stage mean the extra time is much less than double.
Not always. Extra bedrooms generally add more than extra ground floor space, but only up to the ceiling price for your street, and a good open plan kitchen can be the stronger selling point in family housing. Ask a local agent what your finished house would realistically be worth before committing.
You can, if the foundations and walls are designed and built for it from the start, which means specifying that to your engineer at the outset. Retro fitting a second storey to an extension that was not designed for it usually means underpinning and often costs more than building both at once.
Tell us about the build or the joinery and we will come out and give you a free quote.
Tell us about your project and we will get you a free quote.