Prefab vs Brick Annexe: An Honest Comparison

Ask two annexe suppliers which is better, prefab or brick, and each will talk up whatever they happen to sell. This guide takes neither side. We are an independent guide and quote-matching service, so our only job here is to help you pick the route that actually fits your situation.

This is for you if you've narrowed the decision down to modular versus traditional and want a straight answer on where each one wins. You'll get the real cost gap, the speed difference, and the honest caveats on warranty and resale that suppliers tend to skip.

The two routes in one sentence each

A prefab or modular annexe is manufactured off-site in a factory, delivered in sections or as a complete unit, and craned or assembled onto a prepared base in your garden. Most of the build happens before it ever reaches you.

A brick-built annexe is constructed on site the traditional way, with foundations, blockwork or brickwork, and a tiled roof, the same as a small house. Everything happens in your garden, over months.

Both can be excellent. Both can be done badly. The difference that matters is which set of trade-offs suits you.

Cost: the gap is real

On price, this is not close. Factory-built annexes are the value benchmark, and traditional brick sits well above them.

• Prefab / modular: roughly £20,000–£70,000 installed. Entry-level studios start around £20,000–£35,000; a well-specified one-bedroom modular annexe runs £40,000–£70,000.
• Brick-built: roughly £60,000–£160,000+. Expect £60,000–£120,000 for a studio and £80,000–£160,000+ for a one-bedroom build at a quality specification.

The important point that neither figure includes: the site-specific extras apply to both routes. Groundworks and base preparation run £3,000–£10,000. Service connections for water, drainage and electricity run £2,000–£8,000, and drainage is usually the constraint that shapes the whole design. A brick build does not escape these, and a cheap modular headline does not include them. Our granny annexe costs guide breaks down every line item, but the rule of thumb is to add 15–25% to any modular headline for a real total, and carry a 10–15% contingency on a brick build.

Where the money goes differently: with modular, the building price is usually genuinely fixed at the factory, so most of your risk lives in the site. With brick, the site is the build, so cost creep, weather delays and specification changes have far more room to bite.

Speed: months versus days on site

This is modular's clearest structural advantage, and it is worth understanding why.

A factory can build your annexe while your planning application is still with the council, because the two happen in parallel. Once permission is granted and the base is cured, installation itself takes days, not months. Families who need a relative moved in before winter, or before a hospital discharge, feel this difference sharply.

A brick annexe cannot start in earnest until permission is in hand and the foundations go in, and then every stage happens sequentially in your garden. Realistically that is several months of on-site work, weather permitting.

So if timing is the pressure, modular wins before you even open the spreadsheet.

Planning: brick reads as permanent

Here the balance tips the other way, at least in sensitive locations.

Most gardens take either route without trouble, and planners judge an annexe on scale, occupancy and neighbour amenity rather than construction method. A modular annexe is completely fine in the majority of settings.

The exception is context. In a conservation area, near a listed building, or in a village where materials matter, a brick annexe that matches the main house reads as permanent, in-keeping development, and that can be the difference between approval and refusal. A lightweight modular box in the same setting invites more scrutiny. If you are anywhere sensitive, brick often clears planning more comfortably.

One thing worth repeating, whichever route you choose: a self-contained annexe almost always needs planning permission regardless of how it is built. Do not let a supplier tell you a factory building sidesteps that. We cover the routes in the annexe planning permission guide, and the Planning Portal outbuildings pages explain why "incidental" garden buildings and living accommodation are treated differently.

Longevity, warranty and mortgageability

This is where you should read the small print, because it is where the honest caveats live.

A reputable modular annexe typically comes with a 10-year structural warranty, and quality manufacturers stand behind that. A well-made modular building will last decades. But "modular" covers everything from a serious insulated timber-frame structure down to a glorified garden room, and the gap in build quality between the top and bottom of the market is enormous. The warranty and the specification sheet, not the brochure photos, tell you which end you are dealing with.

A brick annexe has a longer expected life and, crucially, reads as permanent value to a surveyor. When your house is valued or sold, a brick outbuilding on proper foundations is understood as part of the property in a way a demountable unit may not be.

The honest caveat: some lightweight modular buildings can affect valuation or mortgageability. A surveyor may treat a portable-style structure as a chattel rather than part of the property, which can complicate a mortgage or remortgage and may not add the value you expect. This does not apply to every modular annexe, and a properly founded, permanently connected unit is a very different case from a wheeled cabin. If resale value or future borrowing matters to you, ask the manufacturer directly how their building is treated for valuation, and check the structural warranty is from a recognised provider such as an NHBC-backed scheme or equivalent. For how an annexe of either type affects what your home is worth, see annexe vs extension value.

Thermal performance: closer than you'd think

There is a lingering assumption that brick is warmer. It isn't automatically.

Modern modular annexes are built to current building regulations and can match or beat brick U-values, because factory-controlled insulation and airtightness are easier to get right than the same work done in the rain. A good modular unit is a genuinely warm, cheap-to-run building.

That said, we would not overclaim here either. A poorly specified modular box will underperform a well-built brick one, and vice versa. Ask both routes the same question: what are the floor, wall and roof U-values, and how is airtightness handled? The numbers settle the argument, not the material.

Disruption, foundations and reversibility

Modular means less mess. The bulk of the build happens in a factory, so your garden is a building site for days rather than months, which matters if a vulnerable relative already lives next to the works.

Foundations differ too. Many modular annexes sit on a lighter base or ground screws, which is quicker and cheaper than traditional footings. A brick build needs full foundations, and that is part of why it costs and takes what it does.

Then there is reversibility, which rarely gets discussed and sometimes should decide it. A modular annexe can often be removed or even relocated later. If you think you might sell within a few years, or the annexe is for a specific and time-limited need, a building you can take away is a real advantage. A brick annexe is permanent by definition, for better and for worse.

Prefab vs brick at a glance

Factor	Prefab / modular	Brick-built
Typical cost (build only)	£20,000–£70,000	£60,000–£160,000+
Time on site	Days once base is ready	Several months
Built during planning?	Yes, in parallel	No, sequential
Sensitive / conservation areas	Fine in most gardens	Often reads better
Structural warranty	Typically 10 years	Longer expected life
Resale / mortgage read	Varies; check with maker	Permanent value
Thermal performance	Can match or beat brick	Good, not automatically better
Reversible / relocatable	Often yes	No

Site extras (groundworks £3,000–£10,000, connections £2,000–£8,000) apply to both. Figures reflect the UK market in mid-2026.

How to choose

Strip away the sales talk and the decision comes down to what you are optimising for.

Choose modular if:

• Budget is the binding constraint, or you want the most annexe for your money
• Speed matters, because of a discharge, a season, or a relative's circumstances
• Reversibility appeals, because you may sell or the need is time-limited
• Your garden is a standard setting with no heritage sensitivity

Choose brick if:

• You are in a conservation area, near a listed building, or somewhere materials are scrutinised
• Maximum resale value and surveyor confidence are the priority
• You want the annexe to match the main house exactly
• Permanence, not flexibility, is what you're after

Most families who lead with budget or timing land on modular. Most who lead with location sensitivity or long-term resale land on brick. Neither is a compromise if it matches your priorities.

The honest test before you commit

There is one check worth running whichever way you are leaning, borrowed from our costs guide. If you have talked yourself into a "cheaper" traditional or converted route and the quote keeps creeping upward, stop and price a modular annexe before you sign anything. A brick build or a marginal conversion that drifts past £60,000–£70,000 has quietly entered modular territory, and you may be paying permanent-build money for a compromised result. Get a modular quote as your benchmark even if you expect to reject it. It keeps everyone honest, including you.

To see the specifications and warranties that separate good modular units from weak ones, read our prefab and modular annexes page. For the traditional route and how a matched brick building is designed, see garden annexes. If a valuation or mortgage question is central to your decision, a chartered surveyor via RICS can tell you how a specific building would be treated before you commit.

What to do next

Decide what you are optimising for first, then match the route to it, not the other way round. Once you know whether budget, speed, sensitivity or resale is driving the decision, the choice between prefab and brick usually makes itself.

When you're ready to compare real quotes from suppliers suited to your route, tell us about your project and we'll point you at the right ones for your garden and budget. No pressure, and no obligation to build anything.

Costs and warranty norms reflect the UK market in mid-2026 and vary by region, specification and manufacturer. Valuation and mortgage treatment of any specific building should be confirmed with a qualified surveyor or lender. Figures are for guidance, not quotations.