Planning a house extension or conservatory: from sketch to architect's brief

An extension nearly always starts with a vague sentence: “We'd like a big kitchen-diner that opens onto the garden.” Between that sentence and drawings you can submit to the council, there's a lot of back and forth. Each round costs time, and often fees.

You won't replace the architect, and that isn't the point. The point is to bring them what they'd otherwise spend hours piecing together: the exact state of the house, what you really want to do with the extra square metres, and the constraints of the plot. The same brief works for a conservatory, a single-storey rear extension, a side return or an extra storey.

What the architect needs to know

At a first meeting, an architect or designer is trying to understand five things:

1. The existing house: the house as it is, with its dimensions, its floors and its likely load-bearing walls.
2. The plot: the boundaries, the orientation, the trees, the access, what's buried.
3. The brief: what you want to do in the new room, for how many people, at what time of day.
4. The budget: an honest range, even a wide one.
5. The framework: what the council allows, and any earlier planning permissions.

The four steps below prepare the first three points. The last two you'll discuss with them, but you can do some of the groundwork.

Step 1: survey the existing house

Start with an accurate plan of the house as it is now, with the status “Existing”: it's shown in grey. The full method is in the guide draw a floor plan of your house. For an extension, a few things need extra care:

• The elevation concerned, to the centimetre: openings, sill heights, where the downpipes are.
• The drains and supply pipes leaving that elevation. An extension built over the main inspection chamber means rerouting everything. The guide mapping underground pipes and cables explains how to find them.
• The difference in level between the floor inside and the garden. It decides whether you'll need a step, a ramp or some digging out.
• The walls that look load-bearing: external walls, thick walls in the middle of the house, walls under an upstairs wall. Write your doubts in the Notes field: they're a question for the architect or the structural engineer, not a certainty.

Step 2: turn the brief into real square metres

“A big kitchen” means nothing until you compare it with something. Use your current rooms as a reference: the Room tool works out their area. If your kitchen is 9 m² and you want “twice as big”, you already know the extension needs to add about 9 m² of kitchen, plus circulation space.

Then list what has to fit in the new room, with dimensions measured at home:

• the table and the number of chairs, counting room to push a chair back;
• the sofa, the island, the wood burner, the piano;
• the walkways: you can't move comfortably between an island and a worktop that are too close together.

Draw this furniture as simple rectangles in the future room, using the Area tool set to “Other”. You'll see at once whether 4 × 5 m is enough or whether you need 5 × 6 m. This check takes ten minutes and saves you finding the problem on the construction drawings.

Step 3: place the extension on the plot

Switch the status to “To build”: new walls appear in red. Anything that has to go, such as the section of back wall replaced by a large glazed opening, goes to “To demolish”, in yellow. Many architects use a similar colour code, and anyone reading the plan understands it without explanation.

As you draw, check:

• the distance to the boundaries, which you measure on the plan and will compare with the local rules;
• the garden that's left: a 20 m² extension also eats into the patio, a path, sometimes a tree;
• what runs underneath: show the utilities level, faded;
• the light: a south-facing extension gets lots of sun, lovely in winter but hot in summer; facing north, the light is steadier but the room cooler. Think too about the existing room that ends up behind the extension and loses its window.

Conservatories. A glass roof heats up fast in the sun, and a south-facing conservatory can become hard to live in during summer without shading and ventilation. That's no reason to give up on it, but it's something to raise from the first sketch: orientation, how much of the roof is solid, blinds, opening vents.

Step 4: compare two or three options

Don't stop at your first idea. Create a variant for each option: “Kitchen extension 4 × 5”, “Conservatory on the west side”, “L-shaped wraparound”. Each variant keeps its own history, and your survey of the existing house stays untouched.

To compare, overlay two variants: the second is outlined in purple. The summary gives the floor areas of the existing rooms and of the rooms to build, so you can compare the options on a simple criterion: how many square metres gained for how much garden lost.

If you've added your own API key, the AI assistant can draw a variant from a sentence (“Create a 4 × 5 m extension against the back wall with a sliding door”), and a Gemini key lets you get a photo-style render of the project. These tools help you picture it and talk it through; they don't replace the architect's decisions.

Planning permission and building regulations: the broad picture

The rules depend on the country, the region and the council. They change, and they have exceptions. Keep the principles in mind and check the detail with your local planning authority.

• In England, many single-storey rear extensions are permitted development, so they don't need a planning application. According to the Planning Portal at the time of writing, the extension must not go more than 4 m beyond the rear wall of the original house for a detached house, or 3 m for any other house; it must be no more than 4 m high; and within 2 m of a boundary the eaves can't be higher than 3 m. Extensions and other buildings together can't cover more than half the land around the original house. Larger rear extensions are possible through a prior approval process, but not on designated land. Conservatories follow the same planning rules as other extensions.
• Building regulations are a separate approval: the Planning Portal states that most extensions need it. A conservatory is normally exempt if it's at ground level, single storey, under 30 m², separated from the house by external-quality walls, doors or windows, with its own independent heating, and if its glazing and fixed electrics comply with the regulations. In Scotland, the equivalent is a building warrant, which you need before you start work.
• Elsewhere: Wales, Scotland and Northern Ireland have their own permitted development rules. In Ireland, certain small extensions count as exempted development under conditions set by the regulations, and your local authority can give you a formal declaration on whether your project needs permission.
• Everywhere, other rules can apply: Article 4 directions, conditions on an earlier permission, restrictive covenants, party wall matters, conservation areas, listed buildings.

A few steps save time. Look up the planning history of your house on the council's online planning register: it shows what was approved, and sometimes what was refused. Many councils offer pre-application advice, and a plan to scale makes that conversation far more concrete. If you're relying on permitted development, a Lawful Development Certificate from the council gives you written confirmation. And if a public sewer runs near the extension, your water company may require a build over agreement: ask early. In the UK, you don't have to use an architect to apply for planning permission, but for anything beyond a simple project, a qualified professional is money well spent.

The budget: what makes the price vary

A price per square metre found online says little about your project. What really weighs:

• the foundations, depending on the ground and the depth;
• opening up a load-bearing wall, which needs a steel beam or a lintel sized by an engineer;
• the junction with the existing house: roof, waterproofing, floor levels;
• services to move: drains, water, electricity;
• glazing and joinery, often a big item in a conservatory;
• heating and insulation, to meet the energy requirements in force.

To compare quotes, give every builder exactly the same plan. A share link with “House” shows the house and the project; each builder can measure and ask questions on the plan itself.

What to bring to the first meeting

• The plan of the existing house, ground and first floor, exported as a PDF to scale.
• The plan of the project with the three colours, and the options you're considering.
• The plan of the plot with the boundaries, the trees and the orientation.
• The map of buried services, even incomplete, with what's certain and what's assumed.
• Photos of each elevation and of the inside of the room concerned.
• The list of needs, with the furniture and its dimensions.
• A budget range.
• The planning history and any earlier permissions, if you have them.

What Architektor doesn't do

To avoid any misunderstanding: Architektor is a sketching tool for plans seen from above. It doesn't draw sections or elevations, doesn't do any structural or energy calculations, and its exports aren't planning application drawings. It helps you think, measure, compare and explain your project. The design, the structure and the official application remain the work of the architect, and in some cases a structural engineer.

That's exactly why a good survey is worth the effort: the architect starts from an accurate base, and your time together goes on the project, not the tape measure.