Every way of adding space to a British house has a different cost per square metre, and the ranking is fairly stable: reorganising what you have, then converting space you already own, then building new. Knowing that order is worth more than any individual saving, because most people compare two options from the same rung.
Use what you already own first
Converting a garage is usually the cheapest way to gain a room, because the structure, roof and foundations exist and the work is a floor, insulation, a wall where the door was and the services. A loft conversion is next, since the roof and the floor area exist and you are buying structure, a stair and finishes. A cellar is more expensive because of waterproofing and head height. Only after those does building new make sense on cost alone, and the reason people skip them is usually that they want the space somewhere specific.
Cheap house extensions: if you are building new, the shape is the saving
A simple rectangle with a flat roof, one storey, against an existing wall, with no drainage in the way and no structural opening bigger than a standard beam, is the cheapest extension there is. Every departure costs: corners, a pitched roof, rooflights, a large glazed opening, a side return that needs a valley, a wrap around that turns a corner. Going up rather than out on an existing single storey extension sounds cheap and usually is not, because the foundations below were rarely built for it.
Cheap home extensions: the savings that are real
Keeping the plan simple and the drainage undisturbed. Choosing a flat roof over a pitched one. Reducing the glazing, which is the single most expensive material in a modern extension. Doing the decoration, flooring and sometimes the kitchen fit yourself on a shell finish. Building in a quieter part of the year if your contractor prices for it. And getting the design right before starting, because changes after work begins are the most expensive things in construction.
The savings that are not
Skimping on the foundations or the base, which is invisible and unfixable. Going without building control, which surfaces at sale and costs more to regularise. Choosing the cheapest quote without a specification, which usually means a different specification rather than a better price. Leaving out insulation now to add it later, which cannot be done without opening up finished work. And a conservatory bought as a cheap extension, which is only cheap while it stays a conservatory: the moment it needs a solid roof and year round use it converges on extension cost with none of the paperwork.
A cheaper alternative to extensions: the option most people do not price
Reorganising the space you have, by moving a stair, removing a wall between two poorly used rooms, or converting an integral garage and rearranging around it, frequently delivers what the household actually wanted for a fraction of an extension. It is worth pricing a small structural reorganisation against a small extension before deciding, because they often solve the same complaint and one of them needs no groundworks at all.
Questions people ask about cheapest way to extend house UK
What is the cheapest way to add a room?
Usually converting space you already own: a garage first, then a loft. Both reuse the structure and foundations, which is where a new extension's money goes.
Is a conservatory a cheap extension?
While it stays a conservatory, yes. Once it needs a solid roof and year round use it converges on extension cost without the paperwork an extension ends with.
Is building up cheaper than building out?
Rarely, on an existing single storey extension, because the foundations below were seldom built to carry another floor. That assessment decides it.
Where should I not save money on an extension?
The foundations, the insulation and the building control sign off. All three are invisible when finished, expensive to correct, and the first two cannot be added later without opening up completed work.