EXTENSION COSTS PART TWO: DOUBLE STOREY
In the second instalment of our extension cost series, the Build It Estimating Service’s Adrian Wild sets out indicative prices for popular two-storey additions
Get an idea of what you can expect to pay for your project with indicative prices from the Build It Estimating Service
If you’re after a value-for-money route to adding more space to your property, look no further than two-storey extensions. This style offers a great chance to rebalance interior layouts, for instance by getting the right mix of bedrooms versus bathrooms upstairs whilst integrating contemporary open-plan space on the ground floor.
What’s more, this isn’t simply a case of paying double to get more living area. You’ll actually spend slightly less per square metre of extra space than you would with a single storey extension. That’s largely because the new zone’s footprint, and therefore the price of the foundations and roof, is pretty much the same with either route.
Generated by the Build It Estimating Service, this indicative double-storey extension cost guide assumes that you’re integrating a kitchen on the ground floor, with a master ensuite bedroom above. The gross internal floor area (GIA) of the addition is 50m2. Two of the examples feature a pitched construction, with the roof tying back to the existing building via valleys, while the third is based on a flat roof.
Prices include all the infrastructure (including cooker and socket circuits, hot and cold water supply, and extension of the heating system). Internal doors have been included, but a wide structural opening from the main property into the new extension would be an additional cost – as would internal walls, flooring and the kitchen and bathroom fit-out (all of which can vary significantly depending on specification).
Our standard specification is based on a brick and block cavity wall extension with PVCU windows and standard patio doors. The flat roof would be in torch-on felt, while pitched versions are based on concrete tile.
The optimised spec demonstrates what you might expect to pay for upgrading the project. It’s based on stone and block construction, aluminium windows and bifold doors. The optimised flat roof option features a GRP covering, while the pitched designs include a clay plain tile finish.
You’ll see that the optimised hip-end extension is some 63% more expensive than the basic flat roof option – so an awful lot comes down to your project priorities in terms of style, quality and long-term performance.
All the prices take in the builder’s overheads and profit, as well as VAT. If you went down the DIY route, you might save as much as 50% on total costs; but at the expense of your own time and the risk of missteps along the way. Potential additional costs include architect’s plans, clearing the site and moving services (such as drainage).
Flat roof two-storey extension
Flat roof extensions are often preferred for 1960s and 1970s properties, or to create a striking, contemporary look that contrasts with a heritage home. This style can work especially well where the main house is relatively low and planning concerns limit a multi-storey addition’s ridge and eaves height, as the flat roof will give you maximum space across the entire upper floor.
Apex valley roof twostorey extension
This type of addition has two sloping roof planes that come together at a ridge; so the wall features a triangular top section. Apex valleys are straightforward to construct and easy to waterproof. Extensions of this type work well if you’re trying to match the pitch and scale of the existing property, or where you need a decent roof void (ie for storage or to turn into a feature vaulted ceiling).
Hip valley roof twostorey extension
Hip roofs slope on all four sides, so they are slightly more complicated than apex designs. They’re typically chosen where you want to match a hip on the main house. They’re also a good choice for minimising the visual impact of the addition, and reducing overshadowing of a neighbour’s property and garden. But if you’re after maximum space in the loft, a hip roof may not be advisable.