Plans to plonk a 48-metre-high tower complex into the middle of low-rise Streatham Vale came closer to reality this week, as Mayor Khan's officers waved through a stage-two consultation on the scheme for 237 flats on the Homebase site at Woodgate Drive.

But locals continue to question whether Lambeth Council, and now also the GLA, have really followed their own rulebooks, in piecing together justification for planning permission that they always wanted to grant.

On one key issue in particular - of putting a tall building in a area identified as a protected view by the council's own Local Plan - the GLA report appeared to dodge the question, and also seemed reluctant to back the stance taken by Lambeth officers.

And the council's position, adopted back in March and accepted by a majority of the planning committee, in turn leant heavily on "draft guidance" - a reference to a document that is still not officially in force, and which contains errors and ambiguity.

The Mayor could have refused the plans outright or taken on the case itself, but has opted to leave authority with Lambeth. The four-block development is "acceptable in strategic planning terms, and there are no sound planning reasons for the Mayor to intervene in this case", the GLA's decision says.

That leaves local residents, several hundred of whom objected to the scheme, looking at paying for a judicial review of the plans as their only realistic option for keeping developer Hadley at bay.

This is in spite of the fact that both the London Plan and the borough's Local Plan seem on the face of it to set a high bar to tall buildings - defined as anything over 25m high, outside the South Circular.

The London Plan says they "should only be developed in locations that are identified as suitable in Development Plans [Local Plans]". This isn't the case here. Lambeth's only identified sites are in Waterloo, Vauxhall and Brixton.

Lambeth's Local Plan isn't supposed to contradict the London Plan, but appears to try to water down this requirement. Even so, it still looks to put stringent demands on tall-building schemes that aren't on these greenlit sites.

"Outside the locations identified... there is no presumption in favour of tall building development... the applicant will be required to provide a clear and convincing justification and demonstrate the appropriateness of the site for a tall building."

The GLA's decision acknowledges a host of issues raised by opponents, most of them related to the scheme's height and governed by tall-building policy - from overshadowing and reductions in daylight to rooftop playgrounds, transport overloading and the number of single-aspect flats.

It concludes that the development is "broadly consistent" with policy and "does not give rise to strategic planning concerns". The "public benefits" compensate for any shortfalls and downsides, it says.

But the GLA almost completely side-steps the issue of putting a tower block in the middle of a protected view - a problem that the council also struggled to resolve in its report to the planning committee, resulting in a series of twisted definitions and tortuous logic.

Lambeth's 2021 Local Plan identifies the views west and south-west from Streatham Common, across Colliers Wood, Morden, Rose Hill and Pollards Hill, as one of the borough's ten protected "panoramas", saying: "The objective in identifying these views is to ensure that no foreground or middle-ground development is intrusive, unsightly or prominent in relation to the panoramic view as a whole, or landmark buildings within."

"In assessing proposals the council will seek to protect their general composition and character from harm." It goes on specifically to say that tall buildings must not "adversely impact on strategic or local views", among other requirements.

At the same time, the council's report includes a mock-up of the view from the common that shows almost half of Block C, the tallest of the four blocks, above the horizon.

Monday's GLA report admits that Block C would be "very prominent" in this view. But it doesn't then conclude that its prominence is damaging, or find it contrary to the Local Plan, because it goes on to conflate the issue of the local view with a similar but separate issue - the protection of the conservation area, which includes Immanuel and St Andrew's Church, at the bottom of the common.

March's planning committee meeting also saw some councillors confused by these two overlapping areas. A senior conservation officer in Lambeth's planning department offered an explanation at the time: "So the Local View is about the distant horizon... we also use this view to assess the effect on the Streatham Conservation Area... the conservation area isn't about Morden - the Streatham Conservation Area is about what is in the foreground."

Lambeth's conclusion on the effects of the towers on the conservation area, focusing on the church, was that there would be a "low level of less than substantial harm" - the development would draw the viewer's eye away from the church, but the church and Block C would be somewhat balanced, as viewed from the common, and the harm would "be outweighed by the public benefits".

The GLA initially refers to the two protected areas together but separately: "At consultation stage GLA Officers noted the proposed development would appear in the protected local view corridor and would impact the view of the Immanuel Church which is identified in the Streatham Common Conservation Area Appraisal as a focal point and significant landmark."

But then later in the same paragraph it starts to merge the issues of the "building" - meaning the church - and the "corridor". "While the proposed development would appear in the background of this building in the local view corridor from Streatham Common, it is considered the impact of proposed Block A would not be significant whereby this building would only appear marginally above the tree line and would remain in line with exiting [sic] taller development in this view and would remain below the horizon."

The GLA doesn't make clear why it chooses to refer here to Block A - which is only planned at 23m high, and therefore - because Streatham Vale sits well below Streatham Common - entirely below the horizon, according to the report.

It does then go on to talk about the 48m Block C - but now makes no reference to the "corridor". Its reference to the "view" here becomes ambiguous - it appears to mean the view of the conservation area, since that's the issue it then deals with, or perhaps it means the church as a "landmark building" in the local view. Either way, there's no attempt to talk about the issue of Block C being 20m taller than Block A and therefore obscuring the horizon.

"In relation to Block C which would be very prominent in this view, the tower is sited away from the Church and would not impact the viewers ability to see the church."

For its part, Lambeth's report from early this year did attempt to explain why putting Block C in the middle of the local view wasn't a problem - it just seems not to be an explanation that the GLA was comfortable echoing.

Lambeth's explanation is based on its Supplementary Planning Documents (SPDs) - a set of additions to the Local Plan, one of which deals with protected views. A first draft of the SPDs came out for public consultation in late 2020. They emerged again only in January this year for a second four-week public consultation round.

Lambeth says its SPDs "give additional guidance on some of the policies of the Local Plan, where this is needed. SPDs do not have the same status as the policies in the Local Plan but have been subject to public consultation and are taken into account as material considerations in dealing with planning applications."

However, the Local Views SPD has not yet been adopted, at least according to the decisions database available on the council's web site. The report of the decision to put the draft out for a second four-week consultation, back in January - some three years after its first outing, and coincidentally just in time for the council's report to be put to the planning committee in March - contains a sentence that begins with an important-looking "if".

"If adopted," the decision summary says, "this SPD will be a material consideration in decision-making when planning applications affecting local views are being assessed."

SPDs aren't unique to Lambeth or London - they are widely used by councils to elaborate on or help explain policy, but they can't override existing policy or make policy themselves.

Norwich City Council, for example, notes: "[An] SPD is 'supplementary' to adopted planning policy and must be consistent with it – it cannot create new policies... Before an SPD is formally adopted and used in decision making, the city council must consult the public on a draft version of the document for a minimum of four weeks and must take account of any comments received."

Lambeth's SPD wasn't adopted at the time that it wrote its report, or when the planning applications committee met in March. And the consultation prompted several objections to the Streatham section in particular.

But it's clear that its content was still used in the recommendations made in the planning officer's report regardless, and subsequently also in the decisions made by the committee.

And on top of that, the way in which it was used raises a whole new set of questions, as officers' efforts to justify the council's case involved some strenuous mental and semantic gymnastics.

The "visual management guidance" in the SPD says of the local view: "The value of this view is heavily reliant on the strong, distant horizon which contracts [sic] with the foreground landscape of Streatham Common. New development should not prevent the viewer's enjoyment of common and distant horizon."

Under a heading "foreground and middle ground", it goes on: "Given the width of the horizon and the low lying nature of the land in the middle ground it is unlikely that middleground development will harm the viewer’s ability to appreciate the horizon". And under "background": "Development on the distant horizon has the potential to enhance the view by announcing otherwise unidentifiable distant locations."

Lambeth's report to the planning committee chooses not to mention the local view at all in its executive summary. Like the GLA report, it talks about the view of the conservation area, and the church, and most of the other relevant planning issues - but not about the protected view.

But further down in the detail it does aim to justify the tower's place in the landscape - by claiming that the vertical tower would reinforce the horizontal composition of the view. And that a building that obscures the horizon won't change the view of the horizon.

Block C would be "clearly visible" in the view, but "the proposed height and proportions of Block C are relatively modest when seen within the wider and extensive panorama view. As such, Block C would not undermine the strong layered horizontal composition of the view comprising the Common, foreground trees, buildings along the Common’s edge and the horizon."

"Instead, the distinct vertical element punctuating the horizon would help to reinforce and compliment [sic] the views [sic] horizontal composition. The open expanse of the Common, vast sky and horizon would be unchanged and would remain the dominant features in the view."

"As such, officers are satisfied that the proposed development would be in accordance with views guidance in the draft SPD," it says.

The document then suggests that the tower would be in the background of the view - even though the development site is less than 1km from the viewing point, in a straight line, while Morden town centre is well inside the limits of the view at around 6km away.

Because the site - opposite Streatham Common station - is "distant" from Streatham Common itself, the tower benefits from the guidance on "background" developments in the SPD, it claims.

"Block C is located adjacent to Streatham Common Railway Station, a well-used transport node on Streatham Vale. The elevated height of Block C would announce this distant location and as such be consistence with that guidance."

And because it has now been redefined as a "background" development, that can also mean that it doesn't fall under the Local Plan's obligation on foreground and middleground schemes not to be "intrusive, unsightly or prominent in relation to the panoramic view as a whole".

"In conclusion," the officers' report says, "the development causes no harm to local views and complies with LLP [Local Plan] Policy Q25."

This argument that the development is in the background of the view is patently incorrect, but is clearly an important stepping stone towards the target conclusion, of "no harm".

Still, this argument wasn't maintained. In the planning committee meeting in March, officers went for a different argument, telling councillors - in a direct contradiction of the report - that the development was in the middleground of the view.

And they relied on a part of the SPD text that wasn't mentioned in the report, in trying to give an explanation for how the report's authors came to their conclusion.

"Given the broad nature of the horizon and the low-lying nature of the land in the middleground, it is unlikely that middleground development - which would be this proposal - will harm the viewer's ability to appreciate the horizon," a senior officer told the meeting, in reponse to a councillor's question.

"So the officer's view is that if this is about the horizon, then even though the scheme is in it, you can still appreciate the broad horizon in the distance - you can still see Rose Hill, you can still see Morden, and all the rest of it. There's no harm to the view. So that's how they've reached that conclusion."

Not only was this not how they reached their conclusion, and not only is it not what's in the report, it also seems to misinterpret the use of "unlikely" in the SPD.

It is unlikely that a randomly-selected new development in Streatham Vale would impede the view of the horizon. Typical developments aren't tall enough, and in fact no building has ever been tall enough, since the church was built.

But for this particular development - which is several storeys more than tall enough - it isn't "likely" or "unlikely". The size of the scheme is now a known, not an unknown, and so it's now a certainty that it will harm the appreciation of the horizon.

Nobody picked up on the inconsistency in the meeting. The planning committee ultimately voted by four to two to support the plans - a unusually close call, local planning watchers say. The committee only "very rarely" goes against officers' recommendations, especially when, as here, the developer has already paid the council for pre-application advice.

Two councillors from Streatham wards sat on the committee. Green Party councillor Scott Ainslie voted against; Labour councillor Malcolm Clark voted in favour. The other vote against came from Martin Bailey, the Labour councillor for Vauxhall.