In Coombe, the Loft Is Not Automatically the Cheapest Room You Can Add

On a London terrace, converting the loft is almost always the cheapest way to gain a bedroom. The received wisdom is sound: the shell exists, the footprint doesn't change, and no garden is lost.

Coombe breaks that logic. Large detached houses on generous plots sit at the top of the loft cost range and nearer the bottom of the extension cost range, which inverts the usual answer.

That's why Coombe architects tend to price both routes before recommending either. The right answer here depends on your roof shape more than your budget.

The Roof Shape Sets the Price, Not the Floor Area

Loft conversions divide into four types, and the gap between the cheapest and the most expensive is roughly fourfold.

A rooflight conversion touches no structure. A dormer cuts out a section of the rear roof. A hip to gable rebuilds the entire sloping end as a vertical wall. A mansard rebuilds the rear slope at around 72 degrees to form a near vertical wall with a flat roof above.

Coombe's problem is that its housing stock rarely offers the cheap option. Large detached houses typically have hipped or crown roofs, and both push you towards the structural end of that list.

The Coombe Hill house Extension Architecture transformed from 200 to 600 square metres illustrates the configuration. It had a traditional crown roof with front and rear dormers, which is precisely the arrangement that rules out a simple rooflight conversion.

What Each Type Actually Costs in London in 2026

Figures vary between sources, so these are consolidated ranges rather than single numbers.

Conversion type	London cost range 2026	Site duration	Structural work
Rooflight or Velux	£18,000 to £48,000	4 to 6 weeks	Minimal
Rear dormer	£45,000 to £75,000	6 to 8 weeks	Moderate
L shaped dormer	£48,000 to £90,000	8 to 10 weeks	Moderate to heavy
Hip to gable	£47,000 to £100,000	7 to 14 weeks	Heavy
Mansard	£62,000 to £165,000	8 to 12 weeks	Heaviest

Two conditions govern the cheapest row. A rooflight conversion needs at least 2.2 metres of existing ridge height above the ceiling joists, and a layout that can take a compliant staircase without wrecking the floor below.

Most large Coombe houses have the height. Fewer have a roof shape that produces usable floor area without alteration, which is where the cost climbs.

Per Square Metre, a Loft in Coombe Sits at the Top of the Range

Across the UK, loft conversions run roughly £2,400 to £4,100 per square metre excluding finishes. London ranges run £2,500 to £4,500, heavily dependent on finish quality, and London generally adds 20 to 40 percent over national figures.

Coombe sits at the upper end for a specific reason. Specification follows property value, and a loft in a house worth several million is finished to a standard that pushes cost per square metre towards the ceiling rather than the middle.

An en suite adds £8,000 to £15,000 on top, and in this market it isn't optional.

Compare That Against a Ground Floor Extension

Published rates for single storey extension work across London and the Surrey border sit somewhere between £2,000 and £3,500 per square metre, and sources disagree meaningfully within that band. A two storey extension works out cheaper per square metre again, because foundations and roof are paid for once and used twice.

Treat that as a range to test rather than a figure to rely on. On a premium Coombe plot, expect quotes at the top of it or above.

	Loft conversion	Ground floor extension
Cost per square metre	£2,500 to £4,500	£2,000 to £3,500 single storey
Garden lost	None	Yes, and on a protected plot that matters
Structural risk	Roof restructuring, existing loads	Foundations, comparatively predictable
Site duration	4 to 14 weeks	8 to 20 weeks
Disruption	Confined to upper floor and stairwell	Kitchen and living space, often unusable
Planning route	Class B, 50 cubic metres detached	Class A, 4 metres or 8 via prior approval
Tree constraints	None	Root protection areas can rule out positions

On cost per square metre the extension usually wins. On everything else the comparison is genuinely close, and three factors decide it.

Three Things That Override the Cost Comparison

Your remaining allowance. Class B roof volume and Class A extension limits are separate allowances. If previous owners consumed the extension depth or the 50 percent curtilage limit, the loft may be the only route left, whatever it costs per square metre.

Your trees. Coombe plots frequently carry Tree Preservation Orders, and the root protection area calculated to BS 5837:2012 can exclude foundations from precisely the position a ground floor extension wanted. A loft has no such constraint.

Your garden. In an area where plot size and landscape setting carry policy protection, giving up garden has a planning cost as well as an amenity one. The loft leaves the footprint untouched.

Those three points are why comparing headline loft conversion cost figures against extension rates rarely settles the question on its own.

The Costs Both Routes Share

Some lines appear regardless of route, and they're the ones most commonly missing from a builder's quote.

Building Control requires a structural engineer's calculation pack for every type of loft conversion, covering floor beams, ridge beam and any dormer or gable structural elements. That starts from around £395 on a simple job and runs to £900 or £2,500 where the roof is being restructured. It isn't optional on even the simplest rooflight conversion.

Building Control application and inspection fees generally land between £360 and £1,200, depending on complexity and whether you use the council or an approved inspector. A party wall surveyor, where one is needed, costs £900 to £1,800 per neighbour.

On the planning side, a full householder application now costs £548 with a Planning Portal service charge of roughly £91 on top. A Lawful Development Certificate for proposed use runs around £274 and is worth having wherever you rely on permitted development. Architectural fees sit at 8 to 15 percent of build cost on either route.

Where Planning Enters the Cost Equation

A mansard always needs planning permission. Rooflight, dormer and hip to gable conversions on a detached house generally fall within permitted development, provided you stay inside 50 cubic metres of additional roof space and don't extend beyond the existing roof plane at the front or above the highest part of the roof.

Coombe has a useful quirk. Much of the area is designated a strategic area of special character, which is a policy designation and does not remove permitted development rights. Conservation area designation does, and parts of Coombe fall inside one.

So two houses on the same road can face different routes, and the cheaper planning route is worth confirming before comparing build quotes. A conversion needing a full application carries the fee, the eight week determination and the design risk that permitted development avoids entirely.

On Value, Be Sceptical of the Percentages

Loft conversions are widely quoted as adding 15 to 25 percent to property value, and extensions similar. Those figures come from national and London wide averages, and Coombe is neither.

In a market where houses already have four or five bedrooms, a sixth adds less than the same money spent on kitchen and living space. That's a judgement about your specific street rather than a percentage anyone can quote reliably, and it's worth putting to a local agent before committing either way.

The Trade Off, Stated Plainly

If your Coombe house has a hipped or crown roof, a generous garden and unspent extension allowance, the ground floor route will probably cost less per square metre and deliver the more useful space.

If your extension allowance is spent, your trees sit where the foundations would go, or you cannot afford to lose garden on a plot where openness carries policy weight, the loft becomes the better project despite the higher rate.

The mistake is treating cost per square metre as the deciding number. In Coombe it's the least important of the three, and the two that matter, your remaining allowance and your trees, can both be established in an afternoon at no cost.