You've got a garage that's become a dumping ground — bikes, boxes, a lawnmower you haven't used in three years. And you need more space in the house. The question is whether to convert what you've already got, or build something new.
It's a genuinely interesting comparison, because the two projects are solving the same problem — not enough living space — but they're doing it in completely different ways, at completely different costs, with completely different implications for your property's value and your daily life during the build.
This guide gives you TCM's honest comparison — including the advice pattern we use when homeowners ask us which route to take, and why the answer is almost never the same twice.
At a Glance: 2026 Comparison (Hertfordshire)
Garage Conversion
House Extension
All figures based on TCM quotes, Hertfordshire and North London, 2025–26. Standard specification. Excludes architect fees, Building Regulations, and VAT.
What Each Project Actually Involves
Before comparing the numbers, it's worth being clear about what you're actually buying with each project. The cost difference is large — a garage conversion is typically two to three times cheaper than a house extension of similar floor area — but the two projects deliver fundamentally different things.
A garage conversion takes an existing structure — the shell of your garage — and transforms it into habitable living space. The work typically involves:
- Insulating the floor (usually a new insulated screed over the existing concrete slab), walls, and roof
- Installing a new internal door from the house (or enlarging an existing opening)
- Replacing the garage door with a window, glazed door, or solid wall
- First and second fix electrics — lighting, sockets, heating connections
- Plumbing if the new space is to include a bathroom or utility room
- Plastering and decoration
- Building Regulations compliance — ventilation, fire safety, thermal performance
What a garage conversion doesn't involve: groundworks, foundations, new roof construction, or any external structural work. That's why it's cheaper and faster than an extension. But it also means you're constrained by the size and shape of the existing garage — you can't make it bigger without planning permission and additional structural work.
The Real Cost Comparison
The headline cost difference is significant, but the comparison is only meaningful if you're comparing projects that deliver the same outcome. Here's the honest breakdown.
| Cost Element | Garage Conversion (single, 18m²) | Single-Storey Extension (20m²) |
|---|---|---|
| Structural / groundworks | None (shell exists) | £10,000 – £16,000 |
| Roof / shell | None (shell exists) | £14,000 – £20,000 |
| Floor insulation / screed | £2,500 – £4,000 | £2,000 – £3,500 |
| Wall insulation | £1,500 – £2,500 | Included in shell |
| Garage door replacement / glazing | £2,000 – £5,000 | £6,000 – £14,000 |
| Internal door / opening | £800 – £2,000 | £2,500 – £5,000 |
| First & second fix electrics | £2,000 – £3,500 | £3,500 – £6,000 |
| Plumbing (bathroom / utility) | £3,000 – £6,000 | £3,000 – £6,000 |
| Plastering & decoration | £2,500 – £4,000 | £3,500 – £5,500 |
| Structural engineer / architect | £500 – £1,500 | £2,500 – £5,500 |
| Building Regulations | £600 – £900 | £900 – £1,400 |
| Total (standard spec) | £15,400 – £29,400 | £48,400 – £82,400 |
| Cost per m² (usable) | £855 – £1,633 | £2,420 – £4,120 |
All figures based on TCM quotes, Hertfordshire and North London, 2025–26. Standard specification. Excludes VAT and planning application fee (£258 if required).
The Value Question: Which Adds More to Your Property?
Here's where the comparison gets more nuanced. A garage conversion is cheaper, but it doesn't always add more value per pound spent. The reason is the garage itself.
In Hertfordshire's family-buyer market, off-street parking is consistently valued — particularly in areas like Borehamwood, Radlett, and Barnet where on-street parking is competitive. When you convert your garage, you're not just adding living space; you're removing a garage. Buyers notice. Estate agents in these areas typically value a garage at £10,000–£25,000 depending on the property type and location. That value doesn't disappear when you convert — but it does shift from "garage" to "additional room," and the net effect on the property's value depends on what the room becomes.
A garage conversion that adds a fourth bedroom to a three-bedroom property typically returns close to its build cost in value uplift — the bedroom count increase is the most recognised value driver in the Hertfordshire family-buyer market. A garage conversion that creates a utility room or home office adds less value, because buyers may not value the trade-off of losing the garage for a room they could have created differently.
From Our Work
TCM's Decision Pattern When Advising Clients
The property type that almost always points to a garage conversion: 1960s–1980s detached with integral garage
When we're surveying a 1960s or 1970s detached house in Borehamwood, Radlett, or Potters Bar with an integral garage — the type where the garage is built into the ground floor of the house with a bedroom above — the garage conversion is almost always the right first move. These garages are typically 18–22m², well-positioned relative to the kitchen and living room, and already connected to the house's heating and electrical systems. The conversion cost is low, the disruption is minimal, and the result is a genuinely useful room. The main question is whether the homeowner needs the parking space — and in most of these properties, the driveway accommodates two cars without the garage.
The property type that almost always points to an extension: 1930s semi-detached with small side garage
The 1930s semi-detached house with a narrow side garage — common in Borehamwood, Elstree, and parts of Barnet — is the property type where we most often advise against a garage conversion and towards a side-return or rear extension instead. The garage is typically 14–16m², which is small for a habitable room, and its position at the side of the house means the converted space has limited natural light and awkward connection to the main living areas. In these cases, a side-return extension — which incorporates the garage footprint and extends the kitchen — delivers far more usable space and a much better connection to the garden. The cost is higher, but the outcome is transformative rather than incremental.
When we recommend both: the double-garage detached house
For detached properties with a double garage — particularly in Radlett, Shenley, and the higher-value parts of Hertsmere — we often recommend converting one bay of the double garage into living space while retaining the second bay for parking and storage. This approach preserves the parking value, adds a genuinely useful room (typically a home office, gym, or playroom), and costs significantly less than an extension. The conversion of a single bay of a double garage typically costs £18,000–£30,000 and avoids the planning and programme complexity of an extension entirely.
Planning and Programme: A Direct Comparison
| Factor | Garage Conversion | House Extension |
|---|---|---|
| Permitted Development? | Yes — if no structural extension to the garage | Yes — if ≤3m rear (semi/terr) or ≤4m rear (detached) |
| Planning required? | Sometimes — conservation areas, Article 4, detached garages in some boroughs | Side extensions, large rears, conservation areas |
| Design stage | 2–4 weeks (Building Regs drawings only) | 6–12 weeks (architect + structural engineer) |
| Building Regs approval | 3–5 weeks (building notice route) | 6–10 weeks (full plans route recommended) |
| On-site programme | 3–5 weeks | 10–20 weeks |
| Disruption level | Low — work contained within garage | Moderate–High — groundworks, rear access, internal wall removal |
| Party wall notice? | Rarely | Usually for rear and side extensions |
| Garden impact | None | Permanent loss of garden area |
| Loss of garage/parking | Yes — unless double garage with one bay retained | No |
Which Is Right for You?
The right answer depends on four things: your property type, what you need the space for, how much you value the garage, and your budget. Here's the framework TCM uses when advising clients.
- Your garage is integral (built into the house) and well-positioned relative to the main living areas — these convert most successfully.
- You need the space quickly and with minimal disruption — a garage conversion can be complete in 5–7 weeks from instruction, compared to 20–26 weeks for a house extension.
- Your budget is limited — a garage conversion at £15,000–£28,000 is accessible to a much wider range of homeowners than a house extension at £42,000–£65,000.
- You have a double garage and can retain one bay for parking — this is the best of both worlds: new living space without losing the parking value.
- You need a home office, gym, playroom, or utility room rather than a bedroom — these uses don't require the same connection to the main living areas that a bedroom does.
- Your garage is large enough for the intended use — a single garage (typically 16–18m²) is sufficient for a double bedroom or home office; a double garage (typically 32–36m²) can accommodate a large open-plan room.
Check Your Planning Position Before Committing
Both projects can qualify as Permitted Development, but the rules differ — and in parts of Hertsmere and Barnet, PD rights are restricted by conservation area designations and Article 4 directions. A Lawful Development Certificate (LDC) application costs £103 and gives you formal confirmation before you commit to design work. For a garage conversion specifically, some councils require a planning application even when no structural extension is involved — it's worth checking with your local authority or asking TCM to confirm your position as part of a free site survey.
Frequently Asked Questions
Not Sure Which Route Is Right for You?
TCM carries out free site surveys across Hertfordshire and North London. We'll assess your garage, your property type, your planning position, and give you an honest recommendation — along with fixed-price quotes for both options so you can compare them properly.