Hammersmith Loft Conversions: What W6 Allows

Under Class B of the GPDO a loft conversion can add 40 cubic metres of roof space to a terrace house and 50 to anything else without a planning application — and in a Hammersmith conservation area it can add none at all, because Class B does not apply there. Those two facts decide most W6 loft schemes before anyone draws anything. Our loft conversions page covers what a build involves, the cost hub breaks the figures down by area, and the estimator prices your own roof.

Hammersmith is W6: Victorian and Edwardian terraces with rear gardens, loft potential and shared party walls. That housing stock is why the loft conversion is the most common thing we are asked about here, and why the same three questions come back every time.

1. The volume limit, and what counts against it

B.1(d) caps the increase in the cubic content of the resulting roof space against the original at 40 cubic metres in the case of a terrace house, or 50 cubic metres in any other case. Most of W6 is terrace, so 40 is the number that binds.

Two things people get wrong about it. It is a measure of volume, not floor area — a shallow roof with a wide footprint can exhaust the allowance while giving you a room you cannot stand up in. And it counts against the original roof, meaning any previous enlargement comes off your allowance, including one carried out by an owner two sales ago. That is worth establishing from the council's records before the design assumes a clean sheet.

Three further limits sit alongside it:

• B.1(b) — no part may exceed the height of the highest part of the existing roof. The ridge is a ceiling, and it is why a hip-to-gable often gains more usable space than raising anything.
• B.1(c) — nothing may extend beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway. This is the front-dormer rule, and it applies everywhere, conservation area or not.
• B.1(e) — a verandah, balcony or raised platform is excluded outright, and so is installing, altering or replacing a chimney, flue or soil and vent pipe. On a terrace with a stack in the party wall, that last one catches more loft schemes than people expect.

2. The conservation areas are the real gate

B.1(f) removes Class B entirely where the dwellinghouse is on article 2(3) land — which includes conservation areas. Hammersmith has four: Brackenbury Village, Ravenscourt Park, Stowe Road and Brook Green.

Inside them there is no permitted development route for a roof addition at all. Not a smaller one, not a discreet one at the back: a planning application, every time. That is not a reason to give up — dormers are granted in conservation areas regularly — but it changes the programme by eight to twelve weeks and it changes what the design has to argue. We have written that case up separately in the conservation-area loft guide.

The practical step is the same either way and it takes ten minutes: find your address on the council's own map at lbhf.gov.uk/planning and establish the designation before a drawing is commissioned. A scheme designed for permitted development and then discovered to be in Brackenbury Village is a redesign, not an adjustment.

3. The Article 4 does not do what people think it does

This is the most common misunderstanding we hear in W6, and it costs people money in both directions.

Hammersmith & Fulham has a borough-wide Article 4 direction in force since 26 April 2018. It removes permitted development rights for basement development, everywhere in the borough. It does not touch roofs.

So: if you are digging, that is a planning application wherever you are. If you are converting a loft and you are outside a conservation area, the Article 4 is irrelevant to you — and we have met homeowners who abandoned a perfectly permitted scheme because they had been told the borough had "removed permitted development", which is true of one thing and not of theirs.

4. The conditions that shape the detailing

Class B permits the work subject to B.2, and these are the ones that change what gets built rather than whether it is allowed:

• B.2(a) — materials must be of a similar appearance to the existing exterior. On a Hammersmith terrace that usually means slate or a close match, and it is the condition a cheap tile choice fails.
• B.2(b)(i) — other than on a hip-to-gable, or where the roof joins a rear or side extension, the eaves of the original roof must be maintained or reinstated, and the edge of the enlargement nearest the eaves must be not less than 0.2 metres from them, measured along the roof slope. That 200mm set-back is why a well-detailed dormer sits back from the gutter line instead of springing off it.
• B.2(c) — any window in a side elevation must be obscure-glazed, and non-opening unless the opening parts are more than 1.7 metres above the floor of the room. On a terrace, that is the flank window onto the neighbour.

5. What to settle before you commission drawings

1. The designation. Conservation area or not. Everything else follows from it.
2. The original roof volume, and whether any of the 40 cubic metres has already been spent by a previous owner.
3. The party wall position. Shared walls are the norm in W6 and steels usually bear on them, which brings the Party Wall etc. Act 1996 into the programme — see the party wall cost guide.
4. Which type the roof actually suits, which is a structural question before it is a taste one: which loft conversion type and hip-to-gable set out the options.

On the jobs we take in W6 we are the builder, not a marketplace: one contract, one named project lead, and no introducer fee stacked on the build cost. If you want a range for your own roof, the estimator prices it stage by stage, and it will tell you what it cannot know yet rather than guessing.