If you ask five builders this question you will probably get five different answers, mostly because they are each telling you about the type of project they build. So here it is without any bias towards one option over another. Garage conversions, loft conversions, and extensions all add space, and they all cost different amounts for very different reasons.

Key Topics Covered

  • Why cost per square metre is not the full picture
  • Garage conversions and where the savings actually come from
  • Loft conversions and what pushes the price up or down
  • Why extensions cost more but often add more value
  • The trade off between cost and what you actually get
  • What decides which option is right for you
  • How Rosebrick helps you weigh it up properly

Why the Cheapest Option on Paper Is Not Always the Cheapest Overall

Before getting into the numbers, it is worth saying that cost per square metre only tells you part of the story. A cheap way to add fifteen square metres of space you do not really need is not better value than a more expensive way to add the ten square metres your family actually uses. Cost effective means something different depending on what problem you are trying to solve.

With that said, here is how the three main options stack up.

Garage Conversions

If you have a garage that has become a dumping ground for bikes and old paint tins, converting it is usually the cheapest way to gain a usable room. The reason is simple. The walls, roof, and floor slab already exist, so you are not paying for foundations or a new structural shell. Most of the cost goes on insulation, a damp proof membrane, flooring, electrics, and making sure the space meets current building regulations for a habitable room.

A single garage conversion typically costs somewhere between £12,000 and £24,000, and an integral garage attached to the house tends to sit at the lower end of that because it already shares heating and some services with the rest of the property. Compared to a loft conversion or an extension of similar size, this is by some distance the lowest entry cost, and the work is usually done in a matter of weeks rather than months.

The catch is that not everyone has a garage worth converting, and if you are relying on it for parking or storage, losing that space might create a different problem. It is also worth checking whether losing off street parking would affect the value or saleability of your house before committing.

Loft Conversions

A loft conversion sits in the middle price-wise, and the type of conversion makes a big difference to where in that range you end up. A simple Velux conversion, where rooflights are added into the existing roof slope without altering its shape, is the cheapest version and can start from around £20,000. A dormer conversion, which adds a box structure to increase headroom and floor space, generally costs more, often £35,000 to £45,000. Go further into a full mansard or a bespoke design and you are looking at £70,000 or beyond.

The advantage of a loft conversion is that you are working within the existing footprint of the house. There is no need for new foundations, and in most cases you do not eat into garden space at all, which matters if your garden is already on the small side. Many straightforward loft conversions also fall under permitted development, meaning no planning application is needed provided you stay within the size limits.

What you are paying for here is mostly structural work to the existing roof, a proper staircase, and getting natural light and headroom into a space that was never designed to be lived in. None of that is optional if you want a room that actually meets building regulations, so the price reflects real work rather than padding.

Extensions

A single storey rear extension is usually the most expensive of the three, generally running from around £1,800 to £3,000 per square metre depending on specification and location, and a double storey or wraparound extension can comfortably pass £100,000 once you include finishes.

The reason it costs more is straightforward. You are building an entirely new structure from the ground up, complete with foundations, external walls, roof, and full first fix and second fix throughout. There is no existing shell to work with, so every part of it is new.

What you get for that money is different in kind, not just in size, from a garage or loft conversion. Ground floor space, particularly a larger kitchen and living area, tends to be what buyers respond to most strongly when a house eventually goes on the market. Estate agents and surveyors quite consistently say that ground floor living space adds more in real pounds than the equivalent square footage upstairs, even though the percentage increase in value can look similar on paper.

So Which One Actually Is the Most Cost Effective?

Genuinely, it depends on what you are trying to fix. If you need extra space quickly, without much disruption, and you have a garage you rarely use, converting it is very hard to beat on cost. If you need a bedroom or an office and your garden is precious to you, a loft conversion gives you space without touching outdoor land, and a Velux version keeps costs down if you do not need the extra headroom of a dormer. If what you actually want is a bigger kitchen, more room to entertain, or a completely different way of using your ground floor, an extension is the only option that really delivers that, and it tends to be the one that adds the most in absolute value when you eventually sell.

There is also nothing stopping you doing more than one of these over time. Plenty of homeowners start with a loft conversion for an extra bedroom and follow it a few years later with a rear extension for the kitchen, once budget allows.

What Actually Moves the Price Within Each Option

Whichever route you take, a few things consistently affect the final cost. Plumbing and drainage add cost wherever they go, so a garage conversion or loft conversion that includes a shower room will cost more than one that does not. Structural complexity, such as a dormer that alters the roof shape rather than a simple Velux, always costs more than the simplest version of the same project. Specification, meaning the quality of flooring, fittings, and finish you choose, can move the final bill by thousands regardless of which type of project you are doing. And planning permission, where it is required, adds both time and professional fees on top of the build cost itself.

How Rosebrick Helps You Work This Out

At Rosebrick Developments we do not push one type of project over another because it happens to be what we are best set up to build. We look at your house, your budget, and what you actually need the extra space for, and give you a straight comparison of what each option would realistically cost and deliver for your specific property.

We work across Mansfield, Nottinghamshire, South Yorkshire, and Derbyshire, and we would rather talk you out of an expensive extension you do not need than sell you one, if a garage conversion or loft conversion would solve the same problem for less.

Weighing up your options for adding space to your home? Contact Rosebrick Developments today.