Process12 min read20 February 2026

First-Time Renovator's Guide to Home Extensions

Never extended a house before? This step-by-step guide covers everything from your first sketch to moving the furniture back in — with no jargon and no assumptions.

Step 1: Work out what you actually need

Before you think about builders or architects, spend a week writing down every frustration with your current home. Not enough kitchen worktop space? No downstairs toilet? Nowhere to put coats and shoes? These pain points define your brief. Do not start with a solution ('I want a rear extension') — start with the problem ('We need more kitchen and living space'). The solution might be an extension, a loft conversion, a reconfiguration of existing rooms, or a combination. A good designer will identify the best approach. A bad one will just draw what you asked for.

Step 2: Understand the planning rules

Most single-storey rear extensions in Dorset fall within Permitted Development rights, meaning you do not need planning permission. But PD has strict rules: maximum depth of 4m (terraced and semi-detached) or 8m (detached), maximum eaves height of 3m, and the extension cannot be higher than the existing house. If your plans exceed PD limits, you need a Householder Planning Application — this takes 8 weeks and costs £258. Conservation areas (Christchurch, Wimborne Minster, Swanage) have additional restrictions. Do not assume — check before you commit.

Step 3: Set a realistic budget

The build is typically 60–70% of the total project cost. On top of the builder's price, budget for: design fees (£250–£2,500 depending on complexity), structural engineer (£400–£800), building control (£400–£800), party wall surveyor if attached (£700–£1,500 per neighbour), kitchen or bathroom fitout, flooring, decoration, landscaping to repair the garden, skip hire, and a 15% contingency. A common mistake is spending the entire budget on the build and having nothing left for the kitchen or flooring. Write a complete budget before you sign a building contract.

Step 4: Hire the right people in the right order

The sequence matters. First, hire a designer or architect to draw your plans. Second, appoint a structural engineer to specify steels and foundations. Third, submit for building control approval. Fourth, get three builder quotes based on identical drawings and specifications — never compare quotes from different designs. Fifth, appoint the builder and agree a start date. This process takes three to five months from first sketch to builder on site. Rushing leads to mistakes, and mistakes in construction are expensive to fix.

Step 5: Survive the build

A single-storey extension typically takes 10–14 weeks on site. The first three weeks are the most disruptive — foundations, drainage, and structural openings. Expect dust everywhere, no use of the affected rooms, and noise from 8am. Set up a temporary kitchen elsewhere in the house (a microwave, kettle, and mini fridge in the living room). Communicate weekly with your builder — a five-minute site meeting every Monday prevents small misunderstandings from becoming expensive problems. Keep a photo diary of the build; it is useful for snagging and a nice record to look back on.

Written by the PlanBuildCo team

9 years designing extensions and renovations in Poole, Dorset.

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