Image copyright Gary Nash Image caption The new Troubadour in White City is designed to be taken down and rebuilt elsewhere if necessary

Some will be permanent - others may survive for just a couple of years. Either way, London's in the middle of a theatre construction boom. So why is it happening? And doesn't London have enough theatres already?

People come from around the world to enjoy theatre in London. But there are other cities with far more theatre buildings.

The authoritative Theatres Trust reckons there are currently 263 theatres in London. It's about the same number as Tokyo, whereas Paris has around 350. New York tops the list with well over 400.

Producers believe more tickets could be sold in London. But first they need more places to originate shows in and to transfer existing shows to. It's hard (and very expensive) to rent a West End theatre to bring in a hit - whether from a non-commercial London theatre or from Manchester or Paris.

Image copyright Mark Senior Image caption Dino Fetscher and Daisy Boulton in Torch Song which is the Turbine's opening play

Partly that's what lies behind the boom in London theatre construction.

But there's another factor. Property developers with a commercial interest in London's rapidly changing skyline find theatre useful. It attracts punters to areas being promoted as desirable new places to shop or eat or even buy a home.

Tristan Baker is co-founder of Troubadour Theatres which has opened two big new theatre spaces in London. One's on part of the former BBC site at White City and the other's at the old Fountain Studios in Wembley.

In both cases there are two auditoriums with up to 2000 seats. Each area is undergoing extensive redevelopment and eventually the theatres are planned to come down.

"But the buildings are 97% recyclable," Baker points out. "They're built to a high spec and they have great facilities for professionals and for the public. But whether it's after 18 months or five years we can move the building elsewhere. That's what happened at King's Cross where all this started."

Troubadour's production arm Runaway staged live versions of The Railway Children at Waterloo Station and then at King's Cross, including a full-size steam train. That led to building new theatres at King's Cross for the David Bowie musical Lazarus, to house the Donmar's Shakespeare productions and for the Lin-Manuel Miranda show In the Heights.

Image copyright Soda Studios Image caption An artist impression of how the Boulevard Theatre will be

"It proved we could build large, comfortable theatres which are far more than pop-ups but probably aren't going to be permanent. They cost less for producers to rent than a top West End venue.

"We were approached by the developers Quintain who asked if we could do something similar at their huge regeneration site at Wembley Park. And another mega-developer Stanhope had similar ideas for White City."

Baker says Troubadour now has its eye on other sites to build theatres. "I won't say where but it's not just a London thing. There are sites around the UK where developers see theatre as a real place-maker, boosting a place's profile.

"Transport is always crucial: our two current sites in London have great links to rail and the Tube. But outside London there might be places where people would be happy to drive for half an hour or even more to get there.

"But what counts is content. In London most people don't ask themselves what's on at the Gielgud Theatre: they want to know where the good shows are. So what we say is build it and they will come."

Other theatre openings announced for London include:

New Nimax theatre near Tottenham Court Road station (expected to open 2021)

The former Ashcroft Theatre in Croydon reopens shortly as the Ashcroft Playhouse

An additional home for Sadlers Wells in Stratford, east London due to open in 2022

Nicholas Hytner, who opened the Bridge Theatre in 2017, plans another new theatre near Kings Cross

Just behind the Gielgud stands the formerly sleazy alleyway Walker's Court, long home to the Raymond Revuebar. But Soho is changing and from October the new 165-seat Boulevard Theatre is set to continue the area's progress up-market.

The same name was used in the 1970s and 80s for a theatre on nearly the same site. But Rachel Edwards, the new theatre's artistic director, says the impressive new building will be hugely more ambitious both technically and artistically. It's part of a major upgrade to the area costing some £40m.

The opening show is the London premiere of the musical Ghost Quartet. Edwards says she knew at once that the venue would be perfect for small-scale musicals - but she's also looking forward to programming one off late-night gigs and comedy events.

Image copyright The Turbine Theatre Image caption The theatre takes its name from the nearby former Battersea power station

"I think when you open a new venue like this in the middle of Soho you're dealing inevitably with a whole variety of audiences. We have a great new bar and a restaurant to tempt people in: I think increasingly people want the theatre building to be an event in itself."

Russell Potter of the architects Soda was commissioned to bring the new theatre to life. "There's an architectural revolve which means the whole theatre floor can switch within minutes between seven different configurations. And the balcony also spins separately.

"That's great for productions. But financially too you need to be able to use the theatre in different ways. Maybe it's a conference in the morning then the main show in the evening and a cabaret event late-night. It's not really something London has had in such a great location.

"Soho's the most vibrant area London has and one of its strengths is it can surprise you. So one of our ambitions was that the Boulevard should do the same thing: when you come in you won't know what the set-up will look like.

Edwards says some West End venues can offer a poor experience. "It can be about sitting in a slightly uncomfortable seat and paying £8 for a glass of warm Chardonnay. But I think as new venues open the ticket-buyers inevitably are going to expect more from their evening."

Paul Taylor-Mills has just opened the 200-seat Turbine Theatre, part of another huge new development in London on the old Battersea Power Station site. The area's transformation is remarkable: there's even a new pier to get visitors into central London by boat in 15 minutes. Eventually there'll be a new extension of the Northern line too.

Image copyright Fairfield Halls Image caption The Ashcroft playhouse will be within the Fairfield Halls

"We called it the Turbine partly because we're by the old power station. But also as artistic director I want it to be a home for generating new ideas and for existing plays done in exciting new ways. We're starting off reviving Harvey Fierstein's gay classic Torch Song. We've got a great director in Drew McOnie and a cast which includes Matthew Needham and Dino Fetscher.'

Taylor-Mills was approached by developers to discuss a theatre operation suited to an area of up-market bars and restaurants. "It's part of what a new quarter of London needs. Obviously they're delighted to have an extra 200 customers each evening looking to eat and drink. And it gives us the chance to create an important venue from scratch in a location full of character.

"We're being supported by one of the great West End producers, Bill Kenwright, and we'd love a West End transfer in our first year.

"Look at the role theatre played in the King's Cross development, which I was involved with doing In the Heights. That was only 2015 and people were saying audiences wouldn't go to this weird area behind a railway station. Now it's buzzing every night.

"Around London, in the traditional theatre district and beyond, things are changing. Things like the Turbine are only the beginning."