How to Design an Extension: The 5 Steps from Idea to Drawings

Jun 9
5 min read

You've decided to extend the house. Maybe you're after more kitchen space, an extra bedroom, or somewhere to work from. The next question is how to actually design the thing.

We're a Kettering-based team that designs and builds extensions across Northamptonshire. Here's a practical, no-fluff guide to designing a home extension that works for your house, your budget and the way you actually live. Five steps, in the order we'd take them.

How to design an extension

The best way to design a home extension is to work in five stages: define what you actually need, check what your property can physically take, choose the right type of extension, design for how you'll live in it day to day, then get the drawings done by the right professional. Skipping any of these stages tends to end in regret or rework.

Step 1: Get clear on what you need

Most homeowners start with a solution (a rear extension, a loft) before they've defined the problem. Reverse the order.

Walk through the house and write down what isn't working. Are you short on storage? Is the kitchen too narrow to host? Is there nowhere to work from home? Do the bedrooms run out by the time the third child arrives?

A useful exercise: list the rooms you wish you had, and the rooms you have but rarely use. The mismatch often points at the answer. Sometimes you need a smaller, smarter intervention rather than a large bolt-on. Knocking through internal walls or repositioning a staircase can solve more than people think.

It's also worth asking what you've already got that could be reused. If you've got an unused conservatory, for example, the question of whether you can turn a conservatory into an extension is often worth answering before committing to a fresh build.

Step 2: Check what your property can physically take

Before you draw anything, look at the constraints. These shape every design decision that follows.

The main constraints in a typical UK extension project are:

Plot size and boundaries. Where does the property end? Are there boundary lines you need to stay clear of? Are there easements that affect what can be built on certain parts of the garden?
Drains and sewers. A drain cover sitting in the proposed footprint can change the entire design. We've written a separate piece on whether you can build an extension over a drain cover and what your options are when one's in the way.
Existing structure. Will the existing house take the additional load of a double storey, or is it limited to a single storey? What's the roof structure like? Where do load-bearing walls run?
Trees and ecology. Protected trees, root protection areas, and bat or bird habitats can all rule out parts of the garden.
Conservation areas, listed status and Article 4 directions. These tighten what's allowed on the outside of the building and often dictate materials.
Planning constraints. Permitted development covers many extensions, but not all. If your project falls outside permitted development, you'll need planning permission and the timeline that comes with it.

A measured survey at this stage is worth the money. It shows you exactly what you're working with and stops you designing something that won't fit.

Step 3: Choose the right type of extension

The combination of what you need and what the property can take usually narrows the options to two or three. The main types worth considering for most UK homes:

Single storey rear extension. The most common, lowest-disruption option. Good for kitchens, dining rooms, family rooms. Often falls within permitted development.
Side return extension. Particularly common on Victorian terraced homes where there's a narrow strip alongside the kitchen. Adds floor area without losing garden depth.
Wraparound extension. A side return combined with a rear extension. Bigger gain in floor area, larger project.
Double storey extension. Adds rooms upstairs as well as down. Usually the best value per square metre, but more disruptive and almost always needs planning permission.
Loft extension. A dormer or hip-to-gable extension uses the roof space. Good for an extra bedroom or office without losing garden.
Garage conversion. If the garage is no longer used as a garage, converting it is often the cheapest way to add usable space.

Many projects combine two of these. A rear extension plus a loft conversion is a common upgrade for a growing family.

Step 4: Design for how you'll actually live in it

This is where most extensions go wrong. The extension gets built as a standalone box, with a tacked-on opening into the existing house, and the rest of the home stops working as a result.

A few things to think about up front:

Where the light comes from. North-facing extensions need careful glazing and rooflights to avoid being gloomy. South-facing extensions need shading to avoid overheating in summer.
The flow between rooms. Knocking through an old kitchen wall to open onto the new extension is often the single biggest improvement. The new and old should feel like one space, not two.
Where the doors and windows go. Sightlines into the garden, views from the kitchen sink, where the kids will sit, where the dog will lie. These small decisions shape day-to-day living more than the headline floor area.
Where the services run. Plumbing, drainage, gas and electrics all have to go somewhere. Plan their routes early or you'll end up with awkward bulkheads and boxed-in pipes.
What the existing house needs. A new extension often exposes how tired the original house has become. Budget for a refresh of the connecting rooms.

Step 5: Get the drawings done by the right person

Three main routes for getting an extension designed:

An architect. Best for complex, design-led or sensitive projects. RIBA-chartered architects charge a percentage of the construction cost. The right call for unusual sites or homes where design matters more than budget.
An architectural technologist or designer. Cheaper than a full architect. Produces compliant drawings for straightforward projects. Fine for standard extensions on standard plots.
A draughtsperson or CAD designer. If you've already got a clear vision and just need technical drawings, a draughtsperson can produce planning and building regulation drawings at a lower cost. You'll usually need a structural engineer's input separately.

Whichever route you take, the drawings need to do two things: get past planning, and tell the builder how to construct what you've designed. Beautiful renderings that can't actually be built, or won't pass building control, are wasted money.

Get a quote for an extension in Northamptonshire

If you're at the design stage and want one team to handle the drawings, planning and build, speak to our team. We design extensions around how you actually live, not just around what looks good on a plan. Finance is available through Phoenix Financial Consultants if you'd rather spread the cost.