Side-return extension cost in Harrow (2026)

For a two-storey mid-terrace house of the usual size, before extras such as a new kitchen, bathroom or bifold doors. Including VAT.

Likely cost, all in£45,000 to £73,000

Contingency to hold back£5,500

Budget to have available£78,500

All figures include VAT at 20%. Builders often quote without VAT, so add 20% to any quote that says 'plus VAT' before comparing it with this report.

For this job on a house outside a conservation area: Larger home extension: prior approval needed

• ? checkSide extension width and height. A side extension is permitted development only if single storey, no more than 4 m high, and no wider than half the width of the original house. A side-return infill on a terrace usually passes; a wraparound often does not, because the side and rear parts are judged together.
• ? checkRear projection. Assuming the usual 6 m depth, the size priced here, it is beyond the standard 3 m but within the 6 m larger home extension allowance. That route needs prior approval from the council, with neighbours consulted, and is not available in a conservation area. At 3 m or less, no prior approval is needed.
• ? checkHeight. Single storey: no higher than 4 m, and eaves no higher than 3 m if within 2 m of a boundary. A 3 m flat roof at the boundary is the usual answer on a terrace.
• ? checkHalf the garden. Extensions and outbuildings together may not cover more than half the land around the original house.
• ? checkMaterials. Materials must be of similar appearance to the existing house, except for a conservatory.

The full report checks the planning register at your postcode for conservation areas, Article 4 directions, flood zones and listed buildings nearby, which change the answer street by street. The decision is London Borough of Harrow's: their website has the local validation list and any supplementary guidance on extensions. Fees are the national statutory figures.

From first sketch to sign-off, about 23 to 41 weeks, of which 8 to 14 on site.

• Design and drawings4 to 8 weeks
• Prior approval6 to 8 weeks
• Structural design, building regulations drawings, tender4 to 8 weeks
• Party wall notices4 to 10 weeks
• Build8 to 14 weeks
• Completion and sign-off1 to 3 weeks

How much does a side-return extension cost in Harrow?

A side-return extension on a two-storey mid-terrace house in Harrow typically costs £45,000 to £73,000 all in, including VAT. The building work itself is £35,000 to £59,000; the rest is professional and council fees. Hold back about £5,500 more as a contingency. A high-end finish, a bigger footprint or a new kitchen move the figure up; the table on this page shows the range for each variant and finish.

Do I need planning permission for a side-return extension in Harrow?

For a side-return extension on a house outside a conservation area, at the 6 m depth priced here, the usual answer is: larger home extension: prior approval needed. Up to 3 m deep on an attached house is permitted development; 3 to 6 m uses the prior approval route; a conservation area or an Article 4 direction can change that. Whether your street is covered by one of those is checked against the planning register in the full report.

How long does a side-return extension take in Harrow?

About 23 to 41 weeks from first sketch to sign-off, of which 8 to 14 weeks are on site. Design, drawings and the council's decision take most of the rest. Builders in London are often booked three to six months ahead, which adds to the calendar but not to the cost.

What fees are there on top of the builder's price?

For this job, about £5,000 to £19,500: an architect or designer for the drawings, a structural engineer, building control, any council fee, and party wall surveyors if a neighbour dissents. On top of that, hold back a contingency of about £5,500 for what the ground and the existing house turn up.

Do these figures include VAT?

Yes. Every figure on this page and in the report includes VAT at 20%. Builders often quote without it, so add 20% to any quote marked "plus VAT" before comparing.