Build up or build out: which should you do first?
People usually arrive at this question with a budget that covers one project and a house that could take either. It is a good position to be in, and it is worth a few minutes of thought, because the two options are not competing versions of the same thing.
A loft conversion and a rear extension solve different problems. Picking the one that solves yours is more important than picking the one that adds the most square metres.
They are answers to different questions
Go up when the problem is bedrooms. A loft conversion adds a room with a door on the top floor, usually with space for a shower room, and it does it without touching the garden. That is the answer to a growing family, a lodger, or working from a bedroom you also sleep in.
Go out when the problem is daytime space. A rear extension gives you a kitchen and dining room people can be in together, a place for a table, and a back wall that opens onto the garden. That is the answer to a ground floor that works fine for two people and falls apart when there are five.
If you find yourself saying "we need more room" without being able to say which room, spend an evening working out which of those two describes your evenings. The answer is usually obvious once the question is specific.
Where does the friction actually happen? If it is at 7pm around a table, build out. If it is at bedtime, build up.
What each one costs per room
A loft conversion is typically quoted between £45k and £75k for a rear dormer, which usually buys a bedroom and a shower room. A single-storey rear extension is typically £55k to £110k, which usually buys one large open-plan space.
A two-storey rear extension is the interesting middle case. At £110k to £200k it is the largest number here, but it is also the cheapest floor area per square metre, because one set of foundations and one roof are shared between two floors. If you need both kinds of space and can fund it in one go, it is often better value than doing two separate projects.
Disruption is not the same either
A loft conversion mostly happens above you. Access is from the scaffold for most of the job, and the house is only really opened up near the end, when the new staircase goes in. Most people stay at home throughout.
A rear extension happens through the room you cook in. The new shell goes up outside the existing back wall first, which protects you for the early weeks, but there is a day when the wall comes out and a period afterwards when the kitchen is a temporary arrangement in another room.
Neither is unbearable. But if you have a newborn, or you both work from home, the difference is worth weighing.
The order matters if you will eventually do both
Doing both eventually is common, and there is a sensible order.
Build out first if the extension will change where the stairs or services want to go, or if you will want the loft stairs to land somewhere the extension affects. Planning the ground floor first stops you paying twice.
Build up first if the loft is straightforward and the extension needs drawings, planning permission or a party wall agreement you have not started. The loft can be finished while the paperwork for the extension is still moving.
There is one thing worth checking before either: your permitted development allowance. A large rear extension and a loft conversion draw on different allowances, but previous work by a past owner can eat into both. Our guide on planning permission for a loft conversion explains how the roof volume limits work.
What usually settles it
Your roof. If the height from ceiling joists to ridge is under about 2.2m, a rooflight conversion is out and the loft gets more expensive, which shifts the maths. Measuring this is five minutes with a tape.
Your garden. A short garden makes a deep extension a bad trade. If the garden is the thing your family actually uses, do not build over it to get a bigger kitchen. A side return infill takes none of the garden and is often the better move on a small plot.
Your street. There is a practical ceiling to what any given street will return on work like this. Adding a fourth bedroom to a road of three-bedroom houses usually does more for value than adding a second reception room. Adding a sixth bedroom to a road of four-bedroom houses usually does not.
None of these needs a decision today. They need someone to look at your roof, your garden level and your drains, and tell you what each option would realistically give you. That is what a survey is for, and it costs you nothing to find out. Book one here and we will come and measure up.