First floor extension: building a room on top of what you have

A first floor extension builds upwards on a part of the house that already has one storey: an attached garage, a flat-roofed kitchen extension, a utility at the side. You get a bedroom without losing any garden, and the ground floor is already there.

The catch is that whatever is underneath was almost never built to carry a floor above it.

The structure underneath comes first

Before anyone draws a bedroom, an engineer needs to know three things about the existing single storey part:

• The foundations. A garage or small extension may sit on shallow foundations designed for one storey. A trial hole beside the wall shows their depth and width. If they are too small, they are underpinned or new foundations are added.
• The walls. Many attached garages have walls one brick thick, sometimes with piers. They rarely carry a second floor as they are. The usual fixes are an inner leaf of new blockwork or a timber frame inside the garage, or taking the walls down and starting again.
• The roof and first floor. The existing flat or pitched roof comes off. A new floor structure goes in at the right height to meet the landing of the house.

Approved Document A sets the standard the engineer designs to, and building control checks the calculations. If the investigation shows the garage needs rebuilding from the ground, the job becomes a full two storey extension, and it is better to know that before planning drawings are done.

Planning

Building upwards changes the outline of the house, so planning rules apply as they would to any two storey extension. In England:

• Over a side garage, the result is a side extension of more than one storey, which is not permitted development. You need a householder application.
• Over a rear extension, the limits for more than one storey apply: it must project 3 m or less from the original back wall, keep 7 m clear of the rear boundary, match the roof pitch, and have obscure, fixed side windows upstairs below 1.7 m. Many older rear extensions are deeper than 3 m, so most first floor extensions at the back need an application too.

The fee for a householder application in England is £548. Scotland and Wales have their own rules and fees.

What it costs

A first floor extension over a garage of about 25 m² typically costs £45,000 to £75,000, nationally, including VAT. Expect 20 to 40% more in London and the South East. Strengthening or rebuilding the garage walls and foundations is the main reason a job lands at the top of that range. It is a guide, not a quote.

Add the engineer’s fees, building control and, in England, the £548 planning fee.

Does a prefab upper floor help?

It can. A timber frame or SIPs upper floor is lighter than block walls, which sometimes means the existing structure needs less strengthening. Panels or a finished module can be lifted on in a day, so the room below spends less time with its roof off.

Against that, a crane has to reach the spot, often over the house from the road, and the structure underneath must still be assessed and put right first. The new floor is only as good as what it sits on. If you are also adding a ground floor at the same time, look at a full two storey prefab extension instead, where the foundations and frame are designed together.

Living with it

The room underneath is out of use while its roof is off, and the upstairs of the house loses its wall where the new room joins the landing. Plan where the car, the freezer or the bikes will go if the garage is involved, and agree with your builder how the house will be closed up each night once the wall upstairs is opened.

Fire and stairs

The new room is a first-floor habitable room, so it needs an escape window or a protected route out, and mains-linked smoke alarms on the landing. Access is normally from the existing landing through a new doorway; check early that the floor levels can line up without a step.