Hampstead Architecture and Planning

Haringey

Kitchen Extension Architects in Alexandra Park

Two kitchen extensions of identical floor area can differ in cost by a factor of two, and the reason is almost never the kitchen. It is a short list of decisions taken early — how the ground is dealt with, how far the structure spans, how much of the envelope is glass, and how the new level meets the garden — and knowing which ones move the number lets you spend where it shows.

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The short answer

What makes one Alexandra Park extension cost twice another?

The ground, the spans and the glass — in that order, and none of them is the kitchen. Foundations, drainage and retaining structure can absorb a fifth of the budget invisibly; clear spans and open corners scale sharply; and large high-specification glazing costs several times a solid wall. Spend economically on what you cannot see, deliberately on what you experience, and survey the ground before the brief is fixed.

Planning at a glance

Alexandra Park

Planning at a glance in Alexandra Park
Local planning authorityHaringey
Likely approval routeHouseholder planning application or permitted development, depending on conservation status, listing and property type — confirmed for the exact address before drawings begin
Statutory determination time8 weeks for most householder applications (13 weeks for major applications)
Where most projects startPlanning Route Review — £395 inc. VAT, credited in full against your package if you proceed. Find my package →

Ground is the first cost driver and the one most often underestimated. Foundation depth is set by the soil, by any trees within influencing distance and by what is already in the ground, and the difference between a standard trench fill and a deep or piled solution with a suspended slab is a substantial share of the shell cost. A drainage survey and, where trees are present, an arboricultural assessment at feasibility convert that from an open contingency into a figure — which is worth doing before the brief is fixed, not after.

Structure is the second, and it scales sharply with clear span. A modest opening carried on a single beam bearing into existing masonry is inexpensive. A fully open corner, a long span with no intermediate support, or a structure carrying a level above are each significantly more, because the sections get deeper, the connections get more complex and the temporary works get heavier. Accepting one intermediate column, or a slightly narrower opening, frequently buys back more than it costs in perceived quality.

The envelope is the third, and glass is where budgets quietly leave. Large sliding or pivot units, structural glazing, slim sightlines and roof lanterns are each several times the cost per square metre of an insulated solid wall, and they bring consequential costs — heavier lintels, more onerous thermal compensation elsewhere under Part L, and solar control and shading if the elevation faces south or west. A smaller area of very good glazing in the right place almost always reads better than a larger area of ordinary glazing.

Level change is the fourth and it is specific to a sloping plot. Where the garden falls away, the extension either steps down internally, sits on a plinth with retaining structure and steps outside, or the ground is regraded. Each is a different cost, and retaining structure in particular is expensive, invisible and non-negotiable once required. Establishing the fall across the plot with a measured survey at the outset is what stops a design being priced on an assumption that the ground is flat.

Local analysis

What matters in Alexandra Park

The cost drivers, ranked, and where the money is best spent

Ranked by how much they move the total on a typical project, the drivers run roughly as follows. First, the ground: foundation type and depth, any build-over agreement for a drain, and retaining structure on a sloping plot. These are invisible in the finished room and can absorb a fifth of the budget before anything is seen. Second, the structure: clear spans, open corners and anything carrying a storey above. Third, the envelope: the proportion of glass, its specification and its detailing, plus the roof build-up. Fourth, the services: underfloor heating, a relocated boiler or cylinder, the extract route, and any electrical work triggered elsewhere in the house. Fifth, the finishes and the kitchen itself — which is where owners concentrate their attention and where the marginal pound has the least structural consequence. That ordering suggests a strategy. Resolve the first two as cheaply as the design honestly allows, because money spent there buys nothing you can see: accept the intermediate column, keep the spans sensible, take the drainage survey early so the foundation is designed rather than guessed. Then spend deliberately on the third and fifth, which are what you actually experience: a smaller area of excellent glazing in exactly the right place, good daylight from above where the plan is deep, and a kitchen that is well made. The common failure is the reverse — an ambitious structural gesture that consumes the contingency, followed by value engineering that lands on the glazing specification and the joinery, so the money went into things nobody will ever notice and came out of the things everybody will. A second, quieter point: the cost per square metre falls as the extension gets larger, because the fixed costs of the ground, the structure and the services are spread further. That does not mean building bigger is better — daylight in the middle of the plan usually argues the other way — but it does mean that a very small extension is rarely good value, and that the honest comparison is between a modest extension plus a proper reorganisation of the existing plan, and a larger extension alone.

Building on a plot that slopes away from the house

A falling garden changes the extension from a plan problem into a section problem, and the decision that follows is worth taking consciously because all three options are viable and they cost quite different amounts. The first is to keep the internal floor level with the existing house and let the ground fall away outside, which produces a plinth: the extension sits above the garden at its far end, needs retaining or supporting structure along that edge, and requires steps or a terrace to reach the garden. It gives the best internal continuity and the worst connection to outside, and on a designated address the additional height above the garden is frequently the thing neighbours object to, because it is what they see. The second is to step the internal level down, either once at the junction or as a change within the new room. That follows the ground, reduces the apparent height from the garden, and can produce a genuinely good room — a dining area a step below a kitchen reads as deliberate rather than awkward — but it complicates underfloor heating zoning, it requires careful handrail and threshold detailing, and it makes the whole ground floor less accessible. The third is to regrade the garden itself, cutting into the slope to form a level terrace immediately outside and retaining the higher ground beyond. This gives the best relationship between room and garden and is often the most attractive result, but the retaining structure is a real engineering element with drainage behind it, and where the retained height is significant or the boundary is close it needs designing and may involve the neighbour. What settles the choice is a measured survey establishing the actual fall across the plot, taken before the design begins. Every one of these options is straightforward when it is designed in and expensive when it is discovered.

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Local planning

Planning & conservation context

On an undesignated house a single-storey rear extension can be permitted development under Class A, and prior approval allows a materially deeper one on neighbour notification. Inside a conservation area the allowance is reduced and prior approval is unavailable.

Rookfield Estate is the only nearby designation carrying an Article 4 direction, and the earlier direction remains in force there. Elsewhere the extension position follows from designation alone.

Building regulations govern the cost drivers as much as planning does: Part A for the structure, Part C for ground moisture, Part L for the thermal performance of a heavily glazed envelope and Part F for kitchen extract.

Inside a designation the extension is assessed on how it sits against the host building and, on a landscape-led layout, within its setting — which usually favours taking depth at the rear over filling a visible gap.

Retaining part of the original rear elevation so the addition reads as an addition is one of the more reliable ways to make a large glazed opening acceptable on a designated address.

Where the plot slopes, the height of the extension above the garden is often more contentious than its depth, because it is what neighbours actually see.

For the wider local-authority picture — policies, timescales and application routes — see planning in Haringey.

Illustrative study — street elevation

Services

Architectural services in Alexandra Park

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Nearby

Nearby Kitchen Extension Architects pages

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Architecture in Alexandra Park: FAQs

What actually drives the cost of a kitchen extension?

Roughly in this order: the ground (foundation type and depth, drainage, retaining structure); the structure (clear spans, open corners, anything carrying a storey above); the envelope (how much glass and to what specification); the services; and only then the finishes and the kitchen. The first two are invisible in the finished room and can absorb a fifth of the budget.

Where is money best spent?

On the things you experience. Resolve the ground and structure as economically as the design honestly allows — accept the intermediate column, keep spans sensible, survey the drainage early — and spend deliberately on glazing quality in the right place, daylight into the middle of the plan, and a well-made kitchen. The usual failure is a structural gesture that eats the contingency, then value engineering the glazing.

Is a bigger extension better value?

Cost per square metre does fall as it gets larger, because the fixed costs of ground, structure and services are spread further — so a very small extension is rarely good value. But daylight in the middle of the plan argues the other way, and the honest comparison is usually a modest extension plus a proper reorganisation of the existing plan against a larger extension alone.

My garden slopes. What are the options?

Three, all viable. Keep the floor level with the house and accept a plinth with steps down to the garden — best internally, worst connection outside, and the added height is what neighbours see. Step the internal level down to follow the ground. Or regrade the garden to form a level terrace with retaining structure beyond. A measured survey of the fall settles it before design begins.

Why is retaining structure so expensive?

Because it is a real engineering element rather than a landscaping detail: it has to resist the pressure of the retained ground, it needs drainage behind it to stop water building up, and where the retained height is significant or the boundary close it requires design and may involve the neighbour. It is also entirely invisible in the finished result, which is why it is so often left out of early budgets.

How much difference does the glazing make?

A large one. Sliding and pivot units, structural glazing, slim sightlines and roof lanterns each cost several times an insulated solid wall per square metre, and they bring consequential costs — heavier lintels, thermal compensation elsewhere under Part L, and solar control where the elevation faces south or west. A smaller area of very good glazing almost always reads better than a larger area of ordinary glazing.

Do I need permission for the extension?

On an undesignated house, often not — Class A covers a single-storey rear extension and prior approval allows a deeper one on neighbour notification. Inside a conservation area the allowance is reduced and prior approval is unavailable. Rookfield Estate is the only nearby designation with an Article 4 direction, and there the earlier direction remains in force.

What does a kitchen extension architect in Alexandra Park charge?

We publish fixed-fee packages rather than quoting a percentage before seeing the house, beginning with a route review credited against the design work if you proceed. On a sloping plot we recommend a measured level survey at that stage, because the fall across the garden decides which of three quite differently priced approaches the project is taking.

Discuss a kitchen extension in Alexandra Park

Send your Alexandra Park address, photographs of the rear elevation and garden, and a note of how much the ground falls away if it does. We will identify the cost drivers specific to your plot, recommend where the budget is best concentrated, and design against a realistic figure rather than an optimistic one.

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